Pre-heat your oven to 140C Fan/160C/325F/Gas Mark 3, and line a cupcake tin with cupcake cases
For the cupcake sponge, mix together the butter and caster sugar, ideally using an electric mixer, for 5 minutes until smooth and fluffy
Add the eggs and milk and whisk them in well
Then gently whisk, or fold in, the self raising flour, cocoa powder and baking powder
Divide the mixture between the cupcake cases, and bake them for 25-30 minutes or until a thin skewer inserted in the centre of the cupcakes comes out clean. Put them on a rack to cool completely
For the buttercream, mix the softened butter on it's own for a few minutes. Then add the icing sugar, cocoa powder and milk, and mix until smooth. You can do this by hand, but an electric hand whisk or stand mixer with paddle attachment will provide the best results. If the buttercream is too stiff, you can add a little more milk
Spread or pipe the buttercream on top of the cupcakes. Decorate however you wish
Store in an airtight container in a cool place and eat within 3 days