For the rum cubes, beat the egg whites. Cream butter, sugar, vanilla sugar and yolks. Stir in the remaining ingredients and fold in the beaten egg whites.
Spread the dough on a baking tray and bake at 160 °C for about 30 minutes. Let cool and cut in half.
For the cream, cream the butter with the powdered sugar until fluffy. Stir in the yolks, rum and lemon juice. Spread on one part of the cake, place the other part on top, cut into cubes.
Sprinkle the rum cubes with sugar.