Mix together butter and flour. Add egg yolk, sugar, vanilla sugar, and nutmeg. Knead well. Form a ball and place in the fridge for at least 1 hour. Prepare tart baking tin. Roll out cooled dough, place in the tin, prick with a fork and place in the oven for app. 10 minutes. Put apples on top in the shape of a rosette. Sprinkle with cinnamon (best freshly ground) and some sugar. Bake in the oven for 20 minutes at 180°C.