Preheat oven to 350° and grease a medium with cooking spray. Place sweet potatoes in a large pot and cover with water. Bring to boil then reduce heat and simmer until the sweet potatoes are tender, about 15 minutes. Drain and let cool slightly then transfer to a large bowl.
Add butter to sweet potatoes and use a potato masher, or two forks, to mash. Add milk, brown sugar, eggs, nutmeg, salt and vanilla and stir until smooth. Transfer to prepared casserole dish.
In a medium bowl, add all of the topping ingredients and mix until combined.
Sprinkle the topping mixture over the sweet potatoes and bake until warmed through and golden on top, 25 minutes. Serve warm.