Cake: Preheat oven to 180 degrees C. Grease two 20cm square cake tins with butter and lightly coat the inside with a bit of flour to prevent the cake sticking.
Beat together the margarine and the caster sugar until fluffy, then combine the eggs. Add the flour, cocoa and baking powder then mix well.
Spread half of the mixture into one of the tins. To the remaining cake mixture stir in the chocolate chips.
Put the chocolate chip cake mix into the other tin then put them both in the oven. Check them after 20 minutes.
When the cakes are cooked, let them rest for a while and loosen them on to a cooling rack and cool until they are completely cold.
Icing: Beat the ingredients for the icing together to form an icing which is quite stiff but will spread onto the cake easily.
To Arrange: When cool, spread some icing on top of the cake which you want on the bottom and put the other on top. Spread the icing around the sides first to avoid getting into a complete mess, then do the top. Curl or grate some chocolate over the top.