225g (or 1 cup) unsalted butter, cubed and softened at room temperature
For the ganache layer:
90g (or 3 ounces) dark chocolate
20g (or 5 teaspoons) granulated sugar
60ml (or ¼ cup) heavy whipping cream
15g (or 1 tablespoon) unsalted butter
Instructions
First, make the caramel sauce:
Start by filling your kitchen sink with about 5-cm/2-inches of cold water. Once the caramel is done, you need to dunk the bottom of the pan in this cold water bath to stop the caramel from cooking.
Add the sugar, salt and the water to a medium-sized, light-colored saucepan. The light color of the saucepan makes it easy to monitor the color of the caramel as it cooks. Heat over low heat, stirring with a metal spoon until the sugar has dissolved.
Crank up the heat to medium-high and cook to a golden caramel. Don't stir it at this point. The syrup will first start to caramelize at the edge of the pan. Once this happens, you can gently whisk the caramel to get an even color.
Once you're happy with the color of the caramel, turn the heat down and whisk in the cubed butter. The caramel will sizzle and bubble once you add the butter. This is normal. Whisk gently and crank up the heat to medium-high again.
Carefully drizzle in the cream, whisking continuously. Again, the caramel will bubble and sizzle, so be careful. Once all the cream has been added, turn the heat down, stop whisking and allow to cook for 1 minute. Then remove the pan from the heat and dunk the bottom of the pan into the water bath in the sink.
Pour the caramel into a heatproof container, such as a clean jar, and allow it to cool to room temperature. You can make the caramel ahead, as it will keep, stored in a covered jar in the fridge, for weeks.
Make the chocolate cake layers:
Adjust the oven rack to the lower third position and preheat the oven to 175°C/350°F ( standard oven setting). Butter two 20-cm/8-inch round cake pans. I usually line the bottom of the pan with baking parchment.
In a medium-sized bowl, combine the sugars, oil, eggs and vanilla. Using a mixer fitted with the whisk attachment, mix until well combined, about one minute.
Dissolve the coffee granules in the hot water, then stir in the buttermilk. Set aside.
In a small bowl, stir together the flour, cocoa powder, baking powder, baking soda and salt.
Add the wet and dry ingredients to the egg and sugar mixture. Gently stir the ingredients together, using a rubber spatula or a spoon. Once you’ve incorporated most of the dry ingredients, mix briefly with the mixer until the batter is smooth, about 15 seconds.
Using a spoon, divide and spread the batter evenly over and in the pans. Bake for 40-50 minutes, or until a tester inserted into the center of the cakes comes out clean.
Remove from the oven and allow the cakes to cool to room temperature. Preferably, make the cakes a day (or 3) ahead, so they can cool in the fridge overnight. A cold cake is easier to cut into layers and decorate. If you decide to make the cakes ahead, allow them to cool to room temperature before wrapping them tightly in plastic wrap, then store them in the fridge until needed.
Next, prepare the spicy pumpkin puree which you will use to flavor the buttercream:
Combine the pumpkin puree, butter, brown sugar, cinnamon, and nutmeg in a small saucepan and heat over low heat, stirring continuously with a wooden spoon, until the butter has melted and the sugar has dissolved. Crank up the heat to medium and continue to heat the mixture, stirring constantly, until the mixture comes to a boil. As the mixture is quite thick, you need to look for slow bubbles.
Once the puree is bubbling, set the heat to medium-low and cook the mixture for 5 minutes, stirring slowly but continuously, to allow some of the moisture to evaporate. Then remove the puree from the heat and blitz to a finer puree in a blender or with a stick blender.
Allow the spicy pumpkin puree to cool to room temperature, then chill for about 30 minutes to an hour in the fridge. You can also make this spicy pumpkin puree, which you will mix into your buttercream base, up to 3 days ahead and store it in the fridge in an airtight container until needed.
While the pumpkin puree is chilling, prepare the buttercream base:
Combine sugar and water in a medium-sized saucepan. Heat over low heat, stirring with a metal spoon until the sugar has dissolved and the syrup is clear. Crank up the heat to medium-high and allow syrup to come to a boil.
In the meantime, place the egg whites in a medium-sized, heatproof bowl and mix (with a mixer fitted with the whisk attachment) until the whites are almost able to hold soft peaks.
Once the sugar syrup has come to a boil, clip on a candy (or sugar) thermometer or insert the probe of a multimeter. Cook until the syrup reaches a temperature of 113°C/235°F, then immediately remove from the heat and slowly drizzle the hot syrup into the bowl with the egg whites, mixing continuously to prevent the eggs from scrambling. Don't pour the syrup onto the whisk, or the syrup may splatter against the sides of the bowl (or into your face!). Instead, aim for a spot close to the whisk.
Once all the syrup has been added, keep mixing until the bottom of the bowl feels cool to the touch and the meringue has cooled down to at least body temperature.
Once the meringue has cooled to room temperature, start adding the butter to the meringue one cube at a time, mixing well after each addition. As you're adding the butter, the mixture will lose most of its volume and at some point may even start to look as if it's separating. Don't panic! Just keep mixing and the buttercream will magically come together and become smooth and creamy.
Add the chilled pumpkin puree to the buttercream one tablespoon at a time, mixing well after each addition. Once you’ve added all the puree, mix for another 2 minutes, or until the buttercream looks creamy and smooth.
Use immediately or store in an airtight container or a zipper bag in the fridge for up to a week. To use buttercream that has been refrigerated, first allow to come to room temperature then beat with the mixer until smooth and spreadable again.
To assemble the cake:
Once the cakes have cooled, using a sharp serrated knife to remove any dome from the cakes to level them, then carefully split each cake in half. Make sure to keep your knife perfectly level (place your other hand on top of the cakes; this will give you more control over the knife).
Reserve one of the bottom layers of the cakes for the top of the finished cake. Place a small dollop of buttercream in the middle of a serving plate and place one of the remaining three cake layers on top of it (press the cake layer down onto the plate). The buttercream will prevent the cake from slipping. Place the serving plate on top of a rotating cake stand or a lazy Susan.
Scoop about 200g or 1 cup of buttercream on top of the cake layer and, using a spatula or a spoon, spread it evenly over the top of the cake layer, pushing it towards and a little over the sides of the layer with your spatula. This 'overhanging' buttercream will make frosting the assembled cake a lot easier.
Place the second layer neatly on top of the first and press down lightly. Fit a piping bag with a small open tip and fill it with about 100g or ½ cup of buttercream. Pipe a border of buttercream around the edge of the second cake layer.
Using a spoon, scoop half of the cooled caramel sauce onto the middle of the second cake layer, gently pushing it against the buttercream border you just made.
Place the third cake layer on top, pressing down lightly, top with another 200g (or 1 cup) of buttercream. Using a spatula or spoon, spread the buttercream evenly over the cake layer, pushing the buttercream a little over the sides of the layer again.
Place the cake layer you reserved neatly, flat-side up, on top of the cake. At this point, you should have a cake with four visible cake layers and some overhanging buttercream.
Crumb coat the cake by using a spatula to smooth the overhanging buttercream over the sides of the cake, pushing it over the top edge of the cake. If necessary, add some more buttercream to the sides of the cake until you have a nice thin coating. It’s okay if you can still see the cake underneath. Use the excess buttercream you pushed over the top edge of the cake to thinly frost the top. It doesn't matter if there are any crumbs visible in the frosting. Place the cake in the fridge for 60 minutes to allow the crumb coat to harden.
As the crumb coat sets, make the chocolate ganache:
Chop the chocolate and place in a heatproof bowl. Set aside. In a small saucepan, combine cream and sugar. Heat over low heat, stirring until the sugar has dissolved, then bring the mixture to scalding point.
Pour the hot cream mixture over the chopped chocolate. Allow to rest for about 1 minute, then stir with a spoon until the chocolate has melted and the mixture looks smooth. Add the butter and stir until well combined. The ganache should be thick, but pourable.
Remove the cake from the fridge and drizzle the ganache over the top. Use a spoon to push the ganache a little over the sides of the cake. Place back in the fridge to allow the ganache to set, about 30-60 minutes.
Remove the cake from the fridge a few hours before serving to allow it to come to room temperature. Cake will keep, stored in an airtight container in the fridge, for up to 3 days.
Notes
I chose just to crumb coat the cake, because I really liked the rustic look of it. However, if you want to give the cake a neat finish, there should be enough buttercream to completely cover the cake with buttercream again after the crumb coat has set. You can make the caramel sauce, the chocolate layers, and the spicy pumpkin puree which you will use to flavor the buttercream up to 3 days ahead.
Recipe by The Tough Cookie at https://thetoughcookie.com/2015/10/27/rustic-looking-dark-chocolate-cake-with-pumpkin-buttercream-and-a-caramel-filling/