Preheat oven to 350 degrees F. Spray a 13 x 9-inch baking dish with nonstick cooking spray.
For the sweet potatoes: Place chopped potatoes in a large pot; cover with water. Bring to a boil. Reduce heat and simmer 20 minutes or until tender; drain. Place potatoes in a large bowl. Add sugar, milk, butter, salt, cinnamon, nutmeg, cayenne (if desired), and vanilla. Beat with an electric mixer at medium speed until smooth. Pour mixture into prepared baking dish.
For the topping: Whisk all ingredients in a medium bowl. Sprinkle mixture evenly over potato mixture. Bake for 20-25 minutes, or until golden. Let stand 10 minutes before serving.
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