Preheat oven to 375 degrees. Peel sweet potatoes and cut into a large dice. Place in a large saucepan and cover with cold water. Bring to a boil, then reduce to simmer for about 10 minutes until sweet potatoes are fork tender. Drain and return to pot.
Use a potato masher, ricer, or hand mixer to mash sweet potatoes to desired consistency. Add in remaining ingredients, through salt, and mix well. Pour mixture into a 9 x 9 inch baking dish or casserole dish.
Crumble Topping. To make the topping, toss together the brown sugar, pecans, and flour. Add in the diced butter and use your hands to incorporate. Sprinkle the topping over the sweet potatoes.
Bake, uncovered, for 30-35 minutes until bubbly and heated though. If the topping starts to get to dark during cooking, cover with foil or a lid.
Video
Notes
Cut the sweet potatoes into similar sized chunks so that they cook evenly.
Pour cold water into the pot and then bring to a simmer. Starting in hot water could cook the outside of the sweet potatoes before the inside has a chance to cook.
Return the cooked sweet potatoes to the hot pot to mash. The heat will help evaporate any excess liquid in the potatoes.
For a less sweet dish, reduce the amount of brown sugar in the mash or omit it entirely.
Refrigerate the entire sweet potato casserole with pecan streusel for up to two days before baking. Add additional cooking time to the casserole and make sure the center of the dish is warmed through before serving.