Peel and cube your sweet potatoes. Place them in a large pot and cover with water. Boil for 15-20 minutes until they are fork-tender. Crucial Step: Drain them well and let them sit in the colander for 5 minutes to steam dry. Excess water = watery casserole.
Preheat oven to 350°F. In a large bowl, mash the potatoes. Add the melted butter, milk, sugar, eggs, vanilla, and cinnamon. Beat with an electric mixer until light and fluffy.
Pour the whipped mixture into a greased 9x13 baking dish. Use a spatula to smooth the top perfectly flat.
In a small bowl, combine brown sugar, flour, and chopped pecans. Add the cold cubed butter. Use a fork to mash the butter into the dry ingredients until it looks like wet sand or coarse crumbs.
Sprinkle the topping generously over the potatoes. Bake for 35-40 minutes. The center should be set (not jiggly) and the topping should be golden brown.
Let cool 10 mins before serving.
