Preheat oven to 180 degrees/ gas mark 4. Place 12 cakes into a 12 hole muffin tin.
Place the butter and sugar in a bowl and mix until fully incorporated. Add the eggs, one at a time, beating until everything is fully incorporated. Sift the flour in a separate bowl along with the espresso beating on a low speed until combined.
Divide the mixture between the cases and bake in a pre-heated oven for 20 minutes until well risen. Leave to cool on a wire rack.
Put the butter into a mixer and beat until soft and fluffy. Add the vanilla extract and beat again. Sift half the icing sugar in until incorporated then add the remainder of the sugar and mix again. Add your cocoa powder until well combined.
Then add the espresso a tablespoon at a time, mixing at a medium speed until the buttercream is light and fluffy. Spoon the buttercream into a piping bag and pipe of each of the cakes.