Preheat oven to 375°F. Grease a 2 or 2½ quart casserole dish.

Place the sweet potatoes in a large pot, cover with water, and boil until tender (about 7 minutes). Drain and return to the pot. Add butter, milk, cinnamon, brown sugar, and salt, then mash until smooth.

Transfer the mashed potatoes to the prepared casserole dish and spread into an even layer. If making ahead, allow to cool, then cover and refrigerate.

Place the pan of sweet potatoes in the oven until heated through (5-10 minutes if warm, 30 minutes if refrigerated).

In a medium bowl, combine butter, brown sugar, flour, cinnamon, and salt. Mix in pecans. If making ahead, cover and refrigerate.

Remove the warm sweet potatoes from the oven. Top with half the marshmallows, then sprinkle half the streusel over the marshmallows. Repeat with the remaining marshmallows and streusel.

Place back into the oven for 8-15 minutes until the topping is golden brown. Allow to sit at room temperature before serving.