Preheat your oven to 200°C (180°C fan) and line a 12-hole cupcake tin with cases.
Beat together the butter and sugar until light and fluffy.
Add the eggs and beat until combined.
Fold in the flour and baking powder until just combined.
In a separate bowl, mix the cocoa powder and boiling water together until a smooth paste forms. Add this to your cake mixture and stir until combined.
Divide the batter evenly between the 12 cupcake cases. Bake for 15–18 minutes, or until a skewer inserted into the centre comes out clean. Leave to cool completely.