1. Weigh or lightly spoon cornstarch and flours into dry measuring cups; level with a knife. Combine cornstarch, flours, sugar, and next 3 ingredients (through salt) in a large bowl, stirring with a whisk. Add butter and next 3 ingredients (through eggs); beat with a mixer at high speed until dough is smooth.
2. Spoon dough into a 9 x 5-inch loaf pan coated with cooking spray. Let rise in a warm place (85°), free from drafts, 20 minutes. Cover with plastic wrap, and refrigerate overnight.
3. Remove dough from refrigerator. Let dough rise in a warm place (85°), free from drafts, 2 to 4 hours or until batter is slightly below rim of pan.
4. Preheat oven to 350°.
5. Bake at 350° for 25 minutes. Combine 1 tablespoon water and egg white in a small bowl, stirring with a whisk. Brush loaf with egg white mixture, and cover with foil. Bake an additional 30 minutes. Cool 15 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.