Briefly mix the flour, sugar, margarine, vanilla sugar, baking powder and salt. Add the eggs and stir to a smooth batter on the highest setting.
Divide the dough into 3 equal parts. Mix in the grated zest of one lemon and the juice of half a lemon in the first part; stir in the red grits in the second part; Mix in the cocoa powder in the third part.
Spread the three parts in dots on a baking sheet. Bake the cake for 15 to 20 minutes at 180 ° C until the top is lightly brown.
After it has cooled down, brush the cake with icing (mix the juice of 1/2 lemon with approx. 4 tablespoons of powdered sugar until smooth) and sprinkle the crumble on top.