Preheat oven to 350°F. Pierce sweet potatoes several times all over with a fork, and place on a baking sheet. Bake until tender, about 60 minutes.Once cool enough to handle, peel and add to a large mixing bowl. Whip potatoes with an electric mixer until fluffy and smooth. You need about 4 cups of whipped sweet potatoes.For coarser potatoes, or if you don't have an electric mixer, you can also use a potato masher or a fork.
Add sugar, eggs, butter, evaporated milk, vanilla, cinnamon, salt, and nutmeg to the potatoes. Mix until combined.
Spread the sweet potato mixture evenly in a 9 by 9-inch baking dish.
Add butter, brown sugar, flour, and chopped pecans to a bowl, and mix well. Spread evenly on top of the potatoes. Bake for 30-35 minutes until heated through.
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Notes
Recipe tips:
Make it ahead of time: Turn this into a make ahead sweet potato casserole by cooking your sweet potatoes and whipping them up ahead of time. Store them, tightly covered, in a casserole dish in the refrigerator, for up to a couple of days ahead of time. You can make the topping ahead of time by simply preparing it and storing it to spread on before baking.
Freeze it: Prepare the potatoes as instructed but make the topping on the day you plan to bake it. Thaw the casserole in the refrigerator overnight, and you are good to go!