Preheat the oven to 180 C / 160 C fan / Gas 4. Lightly grease two 23cm (9 in) cake tins and line the base with greased non-stick baking parchment.
Break the eggs into a mixing bowl, add the sugar and whisk until the mixture is pale and thick. Carefully fold in the sifted flour and cocoa powder. Pour the mixture into the prepared tins.
Bake for 25 to 30 minutes, until the sponges are well risen and beginning to shrink away from the sides of the tin. Turn out onto a wire rack to cool completely and peel off and discard the parchment.
Combine the icing sugar and butter and beat until thick and creamy. After you have prepared the icing butter spread it along the top of one of your sponges and attach them both together using this layer. Dust with icing sugar then slice and serve.