Preheat the oven to 180 C / Gas 4 and grease and line two 18cm (7 in) cake tins with baking parchment.
In a large mixing bowl, beat the butter and sugar until pale and fluffy. Add the eggs, one at a time, until creamy. Add the remainder of the ingredients and using an electric mixer, mix until ingredients are smooth. Divide the cake mixture between the two tins.
Bake in the centre of the oven for about 30 minutes or until a skewer inserted into the centre comes out clean. Remove from the oven, remove from the tin and leave to cool completely.
For the buttercream:
Mix the butter, icing sugar and cocoa powder until smooth.
Place the first cake on a serving plate, then spread the filling on top. Sandwich the second cake layer on top. Slice and serve.
Sometimes it is helpful to horizontally cut a little off the top of the bottom cake to form a flat base upon which to spread the filling and place the second cake layer on top.