Dissolve sugar in milk, sprinkle with yeast and let it stand until foamy 10-15 mins
Mix flour, sugar, and salt, make a well in center and cut in butter, mix until coarse texture
Make a well in center again, and pour yolks and yeast mixture, mix well with wooden spoon
Then start kneading and adding milk a little at a time until smooth
Cover and let stand until doubled in size
Preheat oven to 350 F
Divide dough into two portions
Roll flat and fill one with poppy seeds while the other with walnut
Roll and pinch to seal
Poke holes with fork all over dough and brush with egg whites
Bake for 40 mins until golden brown
Cool on rack before slicing