Preheat your oven to 180C/160C Fan/Gas 4.
Grease two loose-bottomed 20cm/8in sandwich tins with butter and line the bases with baking paper.
In a large bowl, combine the butter, sugar, eggs, flour, baking powder, and cocoa. Beat for two minutes until just blended.
Divide the mixture evenly between the tins.
Bake for 25–30 minutes. The sponge is ready when it springs back when pressed lightly.
Leave the cakes to cool in the tins for five minutes, then carefully remove and place on a wire rack to cool completely.
Choose the cake with the best top, then place the other cake top-down onto a serving plate.
