Beat the margarine or butter together with the sugar in a mixer until white. Add the eggs one by one, and continue mixing for a further 3 minutes. Add the milk, then the flour, and the orange peel or vanilla extract. Mix gently. Set aside 1/3 of the dough, transfer it to another bowl, and mix with the cocoa. Dilute with a little milk if needed.
Thoroughly grease a cake mould. Add half the (white) cake mixture, then add the cocoa mixture, then add the rest of the cake mixture.
Carefully dip a thin stick into the cake and move it to give the cake a @[email protected] appearance. Be careful not to mix too much, otherwise the cake will turn out all brown. Bake the cake at 180° C for about 1 hour, depending on your oven.
Prepare the icing as follows: Melt the chocolate in a water bath, or at very low heat. Add the sugar, milk, and cognac, and mix until smooth. Add the pieces of butter, mixing constantly with a wooden spoon, until you have a glossy and smooth icing. Remove from the heat, leave to cool a little, then ice the cake with it.