Preheat oven to 350°.
Combine first 5 ingredients in a small bowl, stirring to form a streusel. Set aside.
Place potatoes in a Dutch oven; add water to cover. Bring to a boil; cover, reduce heat, and simmer 30 minutes or until very tender. Drain well; mash in a large bowl. Stir in 1 cup streusel, granulated sugar, vanilla, egg white, and milk. Spoon into a 2-quart casserole coated with cooking spray; top with remaining streusel. Bake at 350° for 45 minutes.