Preheat the oven to 180 °C. Grease two 20-cm loose-bottomed cake tins and line the bases with baking paper.
1. Mix the butter, castor sugar, flour, cocoa powder, baking powder and eggs in a bowl and stir until smooth and thick.
2. Divide the batter between the prepared tins and spread out evenly with a spatula. Bake for 25 minutes. Remove from the oven and leave the cakes to cool for 5 minutes in the tins before turning them out onto a wire rack. Leave to cool completely.