Rhubarb and Custard Cupcakes
Prep time: 
Total time: 
Fluffy vanilla cupcakes filled with tangy rhubarb goo, topped with a swirl of delicious custard buttercream, rhubarb syrup and crunchy crumbs. Makes 6, but the recipe can easily be doubled.
For the rhubarb goo:
  • 150g (or 5.3 oz) washed and trimmed pink/red rhubarb
  • 1 tablespoon granulated sugar
  • zest of ¼ orange
  • ¼ teaspoon vanilla extract
  • 1½ teaspoons water
For the cupcakes:
  • 60g (or ¼ cup + ¾ teaspoon) unsalted butter, softened at room temperature
  • 60g (or ¼ cup + 2½ teaspoons) granulated sugar
  • 1 large egg (about 50g)
  • 60g (or ½ cup – 1 teaspoon) all-purpose flour
  • ¾ teaspoon baking powder
  • ⅛ teaspoon baking soda
  • pinch of salt
  • 2¼ teaspoons buttermilk
  • ⅛ teaspoon vanilla extract
For the buttercream:
  • 150g (or ½ cup + 2 tablespoons) milk, I used 2% milk
  • 100g (or ½ cup) granulated sugar
  • 20g (or 4 teaspoons) cornstarch
  • 2 large egg yolks
  • 226g (or 1 cup) unsalted butter
  • ¼ teaspoon vanilla extract
For the crumble topping:
  • 39g (or ¼ cup + 1 tablespoon + ¼ teaspoon) all-purpose flour
  • 10g (or 5 teaspoons) rolled oats
  • 1 teaspoon granulated sugar
  • pinch of table salt
  • 25g (or 5¼ teaspoons) melted unsalted butter
Make the rhubarb goo:
  1. Preheat oven to 175°C/350°F (standard oven setting).
  2. Chop the rhubarb into 5-cm/2-inch chunks. Place in the bottom of a medium-sized roasting tray in a single layer and sprinkle with the sugar. Add the orange zest. Next, mix the vanilla with the water and drizzle it oven the rhubarb.
  3. Wrap the roasting tray tightly in aluminum foil and bake in the middle of the oven for 20-25 minutes (now’s the time to make the cupcake batter). Remove from oven and allow to cool a bit so you can handle the tray.
  4. Pour the contents of the roasting tray (broken down rhubarb and liquid) into a strainer set over a small saucepan to catch the liquid. Don’t press down on the rhubarb, or you’ll get rhubarb bits in the syrup. Just let the liquid drain off. Set the liquid aside.
  5. Next, place what’s left in the strainer (the rhubarb) in the bowl of a food processor and blitz to a fine puree. Allow to cool completely.
Make the syrup:
  1. Place the pan with the rhubarb liquid over low heat. Allow to come to a simmer. Reduce by half by letting it bubble gently for 2-3 minutes.
  2. Pour in a heatproof container (a mug) and allow to cool to room temperature.
Make the cupcakes:
  1. Preheat oven to 170°C/340°F (standard oven setting) and line a 6-cup muffin pan with cupcake liners.
  2. In a medium-sized bowl, beat butter until pale and fluffy, about two minutes. Add the sugar and mix until the mixture looks light and fluffy, another 2 minutes.
  3. Crack the egg in a small bowl and whisk with a fork. Add half the egg to the butter and sugar mixture and mix until well combined. Add the remaining half egg and mix until the mixture looks like thick buttercream. The mixture may separate, but if you just keep mixing it will come together again. Make sure to scrape the sides and bottom of the bowl every now and then with a rubber spatula.
  4. Add the flour, baking powder, baking soda and salt and stir with a rubber spatula until somewhat incorporated. Mix briefly with the mixer, about 30 seconds, then scrape the sides and bottom of the bowl again with your rubber spatula and mix for another 5 seconds. Finally, add the buttermilk and vanilla and mix for 30 seconds, or until beautifully combined.
  5. Divide the batter evenly over the lined cups, place in the middle of the oven and bake for 20 minutes, or until a tester inserted into the center of one of the cupcakes comes out clean. Allow to cool to room temperature on a wire rack before frosting.
Make the buttercream:
  1. Combine milk and ¾ of the sugar in a small saucepan. Add the remaining sugar with the cornstarch to a medium-sized, heatproof bowl. Whisk until combined. Add about 4 tablespoons of the milk mixture to the cornstarch mixture and whisk until incorporated. Add the egg yolks and whisk until frothy.
  2. Place milk-and-sugar mixture over low heat and heat gently until scalding. Drizzle the hot milk mixture into the bowl with the yolk-mixture, whisking constantly.
  3. Once all the milk has been added, pour the yolky-mixture back into the saucepan and heat over low heat, stirring constantly with a rubber, heatproof spatula. As the mixture heats, it will thicken. Keep scraping the sides and bottom of the pan and heat the mixture through for another minute, making sure the custard doesn’t catch on the bottom of the pan.
  4. Once heated through, scrape the custard onto a plate and cover immediately with plastic wrap, pressing the plastic wrap directly onto the surface of the custard with your hand (be careful: hot!) to prevent a skin from forming. Set aside to cool to room temperature.
  5. Once the pastry cream has cooled to room temperature, add butter to the bowl of a stand mixer fitted with the paddle attachment. Beat butter until light and fluffy.
  6. Add the pastry cream one spoonful at a time, mixing well after each addition. Once all the pastry cream has been added, add the vanilla and beat for another 2-5 minutes, or until the buttercream looks smooth, creamy and fluffy.
Make the crumble:
  1. Preheat oven to 170°C/340°F (standard oven setting).
  2. Combine all ingredients in a small bowl. Using a wooden spoon, stir until incorporated. Using your fingers, form fine crumbs. Spread the crumbs on a small cookie sheet and bake on a rack in the middle of the oven for 10-15 minutes, or until golden brown.
  3. Remove from the oven and allow to cool completely to room temperature.
  1. Using an apple corer, remove the center of each cupcake. Fill a small piping bag with the rhubarb goo, snip the end of the piping bag, and fill the holes with the rhubarb goo.
  2. Fit a large piping bag with a star tip and fill it with the custard buttercream. Pipe large swirls on top of the cupcakes (you will probably have some leftover buttercream), drizzle with the rhubarb syrup, and sprinkle with crumbs.
  3. Cupcakes keep in an airtight container in the fridge for 3 days. Allow to come to room temperature before serving.
Recipe by The Tough Cookie at https://thetoughcookie.com/2017/05/08/rhubarb-custard-cupcakes/