Cream butter with sugar. Add salt to egg yolks and beat until thick. Scald milk and cool to lukewarm. Add beaten yolks to butter and sugar mixture. Add yeast dissolved in 1 tablespoon of warm water. Add flavoring and mix thoroughly. Add flour alternately with the milk and knead with hand until fingers are free of dough. Let rise for about 2 hours or until double in bulk. Punch down and let rise again for one hour. Place dough on floured board and roll to one-half inch thickness into rectangular shape. Spread with poppy seed mixture and roll like jelly roll, sealing all edges. Place in baking pan and let rise until double in bulk. Bake for 45 minutes in 350° oven.