Preheat oven to 400°.
Combine first 8 ingredients in a large bowl; mash well. Beat egg whites with a mixer at high speed until stiff peaks form. Gently fold beaten egg whites into sweet potato mixture. Spoon mixture into a shallow 3-quart baking dish coated with cooking spray.
Combine hazelnuts and next 3 ingredients (through 1/4 teaspoon ground cinnamon) in a bowl. Cut in butter with a fork until mixture is crumbly. Sprinkle over sweet potato mixture. Bake at 400° for 25 minutes or until topping is lightly browned.