Preheat oven to 350 degrees. Combine flour, brown sugar, pecans, butter, and cinnamon in a small bowl. Stir to form a streusel and set aside. Place potatoes in a sauce pan with water to cover. Bring to a boil, cover, reduce heat, and simmer until very tender. Drain well, place in a large bowl and mash. Add 1 cup of streusel, granulated sugar, vanilla extract, egg white, and evaporated skim milk to the mashed potatoes. Spoon into a 2 qt casserole coated with cooking spray and top with remaining streusel. Bake for 45 minutes.