For the cupcakes

60g good quality (70%) cocoa powder
4 or 5 tablespoons boiling hot water
175g unsalted butter, softened
175g caster sugar
120g self-raising flour
1 level teaspoon baking powder
3 eggs
For the buttercream icing
100g dark chocolate, melted
60g butter, softened
250g icing sugar
Chocolate sprinkles, for decorating