Soften active dry yeast in warm water. Cool milk to lukewarm.
Meanwhile, cream butter; add 1/3 cup sugar and the salt, creaming thoroughly. Add milk to creamed mixture; stir in 1 cup of the flour. Add softened yeast, eggs, and egg yolk; beat well. Stir in remaining flour; beat 5 to 8 minutes longer. Cover; let rise in warm place till a little more than double (about 2 hours).
Stir down; beat well. Cover tightly with aluminum foil, place in refrigerator overnight. Stir down; turn out on lightly floured surface. Divide dough in fourths; set one section aside. Cut the other three sections in half and form each piece in 4 balls (24 in all). Form the reserved sections into 24 smaller balls. Place large balls in greased muffin pans. Poke indentation in top of each and moisten hole slightly with water. Press small ball in each indentation. Cover; let rise in warm place till double (about 1 hour).
Combine egg white and 1 tablespoon sugar; brush tops with mixture. Bake at 375 about 15 minutes, or till done. Serve warm. Makes 2 dozen.