Peel and cube the sweet potatoes, place them in a large pot of boiling water, and cook until fork-tender, about 15-20 minutes. Drain and cool slightly.

Mash the sweet potatoes in a mixing bowl. Add milk, brown sugar, melted butter, beaten egg, cinnamon, nutmeg, vanilla extract, and salt. Mix until creamy.

Preheat oven to 350°F (175°C). Grease a 9x13-inch casserole dish. Spread sweet potato mixture in the dish; cover and refrigerate if prepping in advance.

Prepare the streusel topping by mixing flour, brown sugar, and melted butter until crumbly. Optionally add chopped pecans.

After baking the casserole for 25-30 minutes, remove it, add mini marshmallows, then sprinkle the streusel over them.

Return to oven and bake for another 10-15 minutes until marshmallows are golden and streusel is crisp.

Broil for an additional 1-2 minutes for a toasted effect. Let rest before serving.