Pre-heat the over to 180C/350F/Gas 4. Grease 2, 20cm cake tins.
Blend the cocoa and water in a mixing bowl then leave to cool slightly. Measure all the remaining ingredients into the bowl and beat well until thoroughly blended. Divide the mixture between the tins and level out.
Bake for about 25 minutes or until well risen. Leave to cool in tins for a few minutes before turning out.
For the filling mix together 2oz soft margarine, 6oz icing sugar and 1 tbsp milk. Use half the buttercream to sandwich the cakes together and spread the remainder on top of the cake.