Sift in the flour and cocoa, add the eggs and mix until the mixture is completely combined.
Divide the mixture between to cake tins, then place the cakes into the oven for about 15 minutes, or until cakes have risen and are srpingy to touch.
Turn out onto a wire rack and leave to cool completely.
Cream the icing sugar, cocoa, butter and vanilla together, using a little water if needed. Turn one of the cakes over so that the top is on the bottom. Spread half of the mixture onto it, and then take the other cake and place it on top, so the top is on top. If you have enough butter cream left at the end, spread it round the outsides so that the cake is completely covered, except the bottom.
Leave the frosting to set a little before serving.