For the dough, cream the margarine with the powdered sugar, vanilla sugar and 2 yolks. Sift the flour with the baking powder and stir in the milk. Beat 4 egg whites until stiff and fold in at the end.
Spread the mixture on a greased and floured baking sheet and top with the rhubarb. Bake the cake for about 30 minutes at 180 °C.
For the snow topping, beat the egg whites with the powdered sugar until very stiff. Apply the mixture to the hot cake with a piping bag and bake for another 10 minutes.