Preheat the oven to 170'C.
Mix the cocoa powder with the boiling water to form a thick chocolate paste. Allow to cool.
Combine the butter and the sugar, beating until pale and fluffy. Beat the eggs together and then add to the butter and sugar mixture. Mix thoroughly and then add the chocolate paste. Combine well and then fold in the flour and the baking powder.
Line two 20cm baking tins with butter and baking parchment. Pour the cake batter into the cake tins and bake for 20-25 minutes or until the cakes are cooked through. You can check this by piercing the centre of the cake with a knife or cocktail stick - the cake is cooked when the knife or cocktail stick comes out clean.
Sift the icing sugar to remove all lumps.
Place all of the ingredients in a bowl and leave to melt over a bain-marie. Try not too stir the mixture too much. Once the chocolate and the butter have been melted, remove from the heat and stir to combine.
Spread a little of the icing on top of one of the cakes and then place the second cake on top. Pour the remaining icing over the cake to finish.
«Try making this cake with a white or milk chocolate buttercream icing.»