Preheat oven to 160°C/320°F (standard oven setting) and line the bottom of three 18-cm/7-inch round cake pans with baking parchment. If you’re using non-stick pans, there’s no need to grease the pans.
In a medium-sized bowl, whisk together flour, baking powder and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar until lightened in color and creamy, about 5 minutes on high speed. Use a rubber spatula to scrape the sides and bottom of the bowl every now and then.
Add the eggs one at a time, beating well after each addition (2-3 minutes on high speed). Once all the eggs have been added, beat for another minute, or until the mixture looks smooth and fluffy, like buttercream frosting.
Add half the flour mixture and mix on low speed until just incorporated. Combine buttermilk and vanilla and add half of it to the batter, mixing until incorporated. Repeat with the remaining flour and buttermilk mixtures, ending with the buttermilk mixture.
Add chocolate chips and mix until just incorporated.
Divide the batter evenly over the prepared pans. Place on a rack in the middle of the oven and bake for 40-45 minutes, or until a tester inserted into the center of the cakes comes out clean.
Remove from the oven and allow to cool completely to room temperature before loosening the cakes from the pan by running a knife along the edges of the cakes.
Cakes can be made ahead: once cool, wrap tightly in plastic wrap and place on the fridge for up to 3 days, or freeze up to 2 months.
Make the Swiss vanilla buttercream:
Combine egg whites and sugar in a medium-sized heatproof bowl (or the bowl of a double boiler) and stir with a rubber spatula until incorporated. Set the bowl over a pan of simmering water, making sure the water doesn’t touch the bottom of the bowl, and gently heat the egg mixture, stirring continuously, until the sugar has dissolved. You can check whether the sugar has dissolved by rubbing a bit of the mixture between your fingers; you should no longer feel any sugar crystals. At this point, you can either pasteurize the mixture, or use the mixture as it is.
If you want to pasteurize the egg white mixture, continue to heat the mixture, stirring continuously, until a digital sugar thermometer registers 71°C/160°F.
Remove the egg white mixture from the heat and transfer it to the bowl of a stand mixer or another (cooler) mixing bowl. Using a mixer fitted with the whisk attachment, beat the mixture until the meringue holds stiff peaks and both meringue and bowl have cooled completely to room temperature (you can turn the mixer off once the meringue holds stiff peaks to allow the meringue to cool).
Transfer the meringue to another bowl and add the butter in the bowl of the mixer. Beat on high fora bout 5 minutes, or until creamy and fluffy.
Add the cooled meringue and mix until completely incorporated, 5 minutes on high speed. Scrape the sides and bottom of the bowl every now and then with a rubber spatula. Once the buttercream is smooth and fluffy, add the vanilla and mix for another 2 minutes.
Buttercream is best made right before you need it, but you can make it in advance (though it may separate when you want to use it). Store in an airtight container in the fridge for up to a week. Allow to come to room temperature before use and beat well with a mixer to make it smooth and spreadable again. If the butterceam separates, just keep mixing and it should come together again.
Make the vanilla milk soak:
In a glass, combine milk and vanilla. Stir.
Assembly:
Place first cake layer onto a serving plate. Poke holes into the top of it with a wooden skewer and drizzle with one third of the vanilla milk soak. Top with 200g (or 1 cup + 3 tablespoons) of buttercream, spreading it evenly over the cake layer using an offset spatula.
Place the second cake layer on top of the layer of buttercream and press down lightly to make sure it sticks. Drizzle with half the remaining milk soak and frost with another 200g (1 cup + 3 tablespoons) of buttercream.
Place the last cake layer on top, pressing it down a bit again. Drizzle with the remaining milk soak and frost with a thin layer of buttercream. Use the remaining buttercream to frost the top and sides of the cake with a thin layer of buttercream, scraping the sides of the cake with a bench scraper so that the cake layers show through the frosting. Place the cake in the fridge to chill thoroughly for at least 1 hour.
Once the cake has chilled, make the ganache:
Combine chocolate and cream in a small saucepan. Place over low heat and stir with a rubber spatula until all of the chocolate has melted. You don't want the mixture to get too hot, or it will seize, so pay attention. The ganache should be runny.
Remove the cake from the fridge and use a teaspoon to apply ganache near the edges of the cake so that it drips down the sides of the cake. Pour some ganache on top of the cake and spread evenly using an offset spatula. Place the cake in the fridge to allow the ganache to set.
Cake keeps for a couple of days, stored in an airtight container in the fridge. Allow to come to room temperature before serving.
Recipe by The Tough Cookie at https://thetoughcookie.com/2017/05/02/vanilla-chocolate-chip-drip-cake/