Put the cake tin onto a sheet of greaseproof paper and draw around it. Cut out the circle of paper. Use kitchen paper to grease the tin with a little soft margarine, and then put the circle of greaseproof paper inside the tin. Grease the paper.
Sift the flour and baking powder into a mixing bowl.
Add the margarine, sugar and eggs. Beat everything together with a wooden spoon until completely mixed.
Put half the mixture into another bowl, and then sift the cocoa on top of it. Mix it in well.
Put alternate spoonfuls of the different-coloured mixtures into the tin. Swirl them together a little, and then smooth the top. Put the tin in the centre of the oven and bake for 40-50 minutes.
Take the cake out of the oven and leave it in the tin for five minutes. Turn it out onto a wire rack, peel off the greaseproof paper, turn it over and leave it to cool.
Decorating the cake: Before you start to decorate the cake, cut it in half.
Sift the icing sugar and cocoa into a bowl. Add the hot water, and stir the ingredients together until the mixture is completely brown.
Spread some icing on the one half of the cake, and then place the other half on top to sandwich the icing. Pour the rest of the icing over the top of the cake and neatly spread it to the edges.
Melt the white chocolate in a bowl over a saucepan of simmering water. (Make sure the bowl dosn't touch the water.) Spoon blobs of white chocolate onto the cake and then you can eat.
If your cake does not rise a lot and you cannot cut it in half, make another cake and pretend that is the other half and do the same.