Beat the egg yolks with the sugar until frothy. Melt the butter slowly, add it and stir well. Mix the flour and wheat or potato flour in a bowl and add the butter-sugar-egg mixture and knead everything well (I usually need 400 g wheat or potato flour, because the dough still sticks a little, which is certainly due to the size of the eggs) .
Shape the dough into small balls and press in a little (preferably with a wooden spoon handle). Fill the cavity with jam to taste and bake the cookies at 170 ° for about 20 minutes. It is best to let them cool slightly on the baking sheet as they are very soft and tender when they come out of the oven.