Mix together egg yolks, flour, milk, salt, baking powder and sugar till well mixed. Let batter stand for 10 minutes. Then beat eggs whites until stiff. Gently fold egg whites into batter. Melt butter in frying pan. Pour batter in and fry until cooked. Flip pancake, adding more butter, and continue to cook on other side until crispy. Using 2 forks, tear pancake into pieces and continue cooking briefly. Put powdered sugar on it.