Preheat the oven to 170°C. Line and grease 2 nine inch loose bottomed cake tins and set aside for later.
Cream the sugar and margarine in a bowl until creamy. In a seperate bowl, beat the egg, then stir into the margarine-sugar mixture.
Sift in the flour and hot chocolate powder, and gently fold in. Split the mixture evenly between the 2 cake tins and flatten down with a metal spoon. Place on the middle shelf of the oven for 25 minutes, until a skewer placed in the middle of the cake comes out clean.
Meanwhile, make the icing. Mix the margarine in a bowl until it is creamy and soft. Sift in the icing sugar and stir until combined. Then stir in the hot chocolate powder.
Melt the chocolate for the icing in a microwave at 20 second intervals. Stir into the icing. If you think the icing is a little thick, add a dash of milk to loosen it a little. It needs to be soft and spreadable, but not runny.
When the cakes are ready, take out of the oven and leave to cool completely. Take them out of their tins and check they are not at all warm.
When cool, spread the icing onto the top of one cake and place the second one on top. Melt the chocolate for the topping, and pour onto the top cake. Spread around with the back of a spoon until it is covered.
Leave the chocolate to cool slightly until it isn’t runny anymore, then add the chocolate buttons. When the chocolate layer is almost fully set, score with a sharp knife so that you are able to cut pieces out of it when it is set completely.