Dissolve yeast and 1/2 teaspoon sugar in warm water in a large bowl. Add remaining 1/3 cup sugar, 2 cups flour, potato flakes, milk powder, salt, eggs, and 1/2 cup butter; beat at medium speed of an electric mixer 5 minutes or until smooth. Stir in remaining flour to make a soft dough (dough will be sticky).
Place dough in a greased bowl, turning to grease top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in bulk. Stir dough down; cover and refrigerate 30 minutes. Stir dough down again; cover and refrigerate overnight.
Stir dough down and turn onto a greased 15- x 10- x 1-inch jellyroll pan; press dough gently with lightly floured hands to spread evenly over pan. Cover and repeat rising procedure 1 hour or until doubled in bulk.
Brush milk gently over surface of dough. Combine melted butter, brown sugar, and cinnamon; mix well. Pour mixture evenly over surface of dough. Bake at 375° for 15 minutes or until the top is lightly browned. Cool cake slightly, and cut into squares.