Grease side and bottom of 2 (10-inch) tube pans. Prepare Topping; set aside. In a shallow, medi m bowl, dissolve yeast in warm water. Add 1 tablespoon sugar and 1/2 cup flour; stir to combine. Cover; let stand in a warm place 5 to 10 minutes until foamy. Heat milk and butter or margarine in a small saucepan until melted. Let stand until mixture cools to warm. In a large bowl, beat eggs, egg yolks and remaining 1 cup sugar until pale and frothy. Add cooled milk mixture, salt and yeast mixture. Beat until smooth. Gradually beat in 4-1/2 cups flour. Stir in enough remaining flour to make a soft dough. Turn out dough on a lightly floured surface. Knead dough into a soft, smooth dough. Divide dough in 1/2. Arrange 1 part dough in each greased pan. Cover with a damp cloth; let rise in a warm place, free from drafts, until doubled in bulk, about 2 hours. Preheat oven to 350F (175C). Lightly beat 2 egg whites in a small bowl until foamy. Brush beaten egg whites on top of dough. Evenly sprinkle Topping over dough. Bake 50 to 55 minutes or until a wooden pick inserted in center comes out clean. Cool cakes in pans 3 to 4 minutes on racks. Turn out of pans; cool completely on racks. Frost with Powdered-Sugar Icing, if desired.