kolache recipe

2 packages (1/4 ounce each) Active Dry Yeast ½ cup Sugar (divided) 2 cups Warm 2% Milk (110° to 115°) 4 cups All-Purpose Flour 4 Large Egg Yolks (room temperature) 1 teaspoon Salt ¼ cup Butter (softened) Filling: 2 cups Canned Prune (or poppy seed, cherry, or lemon pie filling) 1 Large Egg White (beaten)