Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line with greaseproof paper the sides of two cake tins roughly a 20cm tin.

Blend the cocoa and boiling water in a large bowl. Add the remaining sponge ingredients and beat until the mixture has become a smooth, thickish batter. This can be easily done with an electric hand whisk. Divide the cake mix equally between the prepared tins and level the surface.

Bake in the pre-heated oven for about 25-30 minutes or until well risen and a skewer inserted into the cetnre comes out clean. Leave to cool in the tins for a few minutes before turning out. Peel off the parchment and finish cooling on a wire rack.

To make the icing, melt the chocolate gently in a bowl set over a pan of hot water, stirring occasionally. Add the butter and stir until the icing has the consistency of thick pouring cream.

Stand the wire rack on a baking tray to catch any drips. Sandwich the cakes together with icing in the middle and use a small palette knife to smooth the icing over the top of the cake.

Allow to time for the icing to cool and set. Decorate how you desire. Keep in a cool place until ready to serve.