Stir together warm milk, yeast, and 1 tablespoon sugar in large bowl. Let stand 5-10 minutes or until foamy. Add 1/2 cup butter, remaining 3 tablespoons sugar, egg yolks, 1 teaspoon vanilla and salt. Mix with electric mixer using dough hook on medium speed until combined. Gradually add 4-1/2 cups flour, 1 cup at a time, and mix on medium speed until a smooth, elastic dough forms, about 5 minutes.