Beat the butter and chocolate mixture for a couple of minutes and then beat in the eggs one by one, alternating them with batches of sugar. Add the vanilla, the flour and baking powder and beat on low to make a smooth batter. Divide the mixture between the sponge tins and bake for 20-25 minutes or until the edges come away from the sides of the tin and a skewer comes out clean. Be careful not to overcook the cake or it will go very brown as there is a lot of sugar in it. Cool in the tin for 10 minutes, then transfer to a wire rack.