(from the fabulous "Cake Days" by the Hummingbird Bakery)
For the sponge
120g unsalted butter, softened
300g caster sugar
2 large eggs
20g cocoa powder
40ml red food colouring (such as Dr Oetker)
1 tsp vanilla essence
300g plain flour
1 tsp salt
1 tbsp white wine vinegar
1 tsp bicarbonate of soda
For the frosting
100g unsalted butter, softened
600g icing sugar
250g full-fat cream cheese (such as Philadelphia)
Coloured sprinkles, to decorate (optional)
One or two 12-hole deep muffin tins
Preheat the oven to 190°C and line a muffin tin with muffin cases.
Using a hand-held electric whisk or a freestanding electric mixer with the paddle attachment, cream the butter and sugar together until pale and fluffy. Break in the eggs one at a time, beating thoroughly after each addition and mixing in the scrapings from the sides of the bowl.
In a separate, small bowl, stir together the cocoa powder, food colouring and vanilla essence to form a paste. Add the paste to the batter, mixing thoroughly until the paste is completely incorporated.
Sift together the flour and salt in another bowl, then add the flour to the batter in two batches, alternating with the buttermilk and mixing thoroughly after each addition. Lastly, in another bowl, mix the vinegar and bicarbonate of soda together by hand and add it to the cake batter, mixing it in until it is fully incorporated.
Spoon the batter into the paper cases, so that they are two-thirds full, using any remaining batter to fill up to four more cases in another tin. Place in the oven and bake for 18-20 minutes or until the sponge bounces back when lightly pressed. Allow the cupcakes to cool for a short while in the tin, then place on a wire rack to cool completely before you frost them.
Using the electric whisk or freestanding mixer with the paddle attachment, and mixing on a low speed, beat the butter and icing sugar together until no large lumps of butter remain and the mixture is sandy in texture. Add the cream cheese and mix together slowly until everything is incorporated, then increase the speed to medium and beat the frosting until it is soft and fluffy.