While dissolving yeast in a bit of water, heat milk to lukewarm. Stir together the egg yolks and sugar and add this to milk together with melted butter, yeast and sifted flour. Last fold in stiffly-beaten egg whites and let rise for 1 hour (in a bowl large enough to allow room for rising). Best when fried in butter, but other shortening may be used, using hot aebleskiver pan. Small slices of apple may be sunk into dough in pan.