Stir the margarine until foamy, add sugar and eggs and stir. Stir in the ground walnuts and the flour mixed with baking powder. Spread the dough on a greased baking sheet and bake in the preheated oven.
In the meantime, mix the icing sugar, coffee and brandy together. Cut the hot cake into squares and cover with the icing while still on the tray. Decorate each piece with a walnut half.
You can also apricot the slices and cover them with chocolate icing, but I think the brandy and coffee icing is the most delicious.