Peel and chop sweet potatoes into 1-inch chunks. Boil them in salted water for about 20 minutes or until tender. Drain well and let them cool a bit.

In a large mixing bowl, blend the cooked sweet potatoes with softened butter, milk, brown sugar, vanilla extract, and salt. Stir gently until smooth; small lumps are fine.

Grease a 9x13-inch baking dish to prevent sticking. Pour the mashed sweet potato mixture into the dish, spreading it evenly.

In a separate bowl, combine flour, brown sugar, melted butter, cinnamon, and nutmeg. Mix until clumpy and sprinkle over the sweet potato mixture.

Preheat oven to 375°F and bake for about 20 minutes until slightly firm and streusel begins to harden.

Carefully remove from oven, sprinkle mini marshmallows on top, and return to the oven for another 10-15 minutes until marshmallows puff and brown.