First stir in the butter at room temperature until frothy, then stir in the sugar, vanilla sugar and the 6 egg yolks individually. Add milk, rum and lemon zest. Finally, briefly stir in the flour sifted with baking powder and the firm egg whites of the 5 (!) Egg whites alternately.
Spread this mixture on a very well buttered and floured baking sheet and cover with halved and pitted apricots (skin side down).
Bake at medium heat for about 35-40 minutes (make a needle test!)