1. Preheat the oven to 180°C/fan160°C/gas 4. Grease 2 x 20cm sandwich tins with a little butter and base line with baking paper. In a bowl, beat together the butter and sugar with an electric hand whisk until light and fluffy.#
2. Add the eggs, 1 at a time, beating after each addition. Sift in the flour, baking powder and cocoa, then lightly fold into the mixture. Add a little milk until you have a dropping consistency, then divide between the tins.
3. Bake for 20-25 minutes or until a skewer inserted into the centre comes out clean. Leave to cool in the tin for 10 minutes, then turn out onto a wire rack to cool completely.
4. Melt the chocolate in a bowl over a pan of simmering water, then cool slightly. In a bowl, beat the butter and icing sugar together with an electric hand whisk until light and fluffy, then fold in the melted chocolate.
5. Spread some icing over 1 of the cakes, top with the other cake and the rest of the icing.