Peel and cube the sweet potatoes, then boil in salted water for 15-20 minutes until tender. Drain and cool slightly.
Mash the sweet potatoes in a large bowl, then mix in granulated sugar, brown sugar, milk, melted butter, eggs, vanilla extract, ground cinnamon, ground nutmeg, and salt until smooth.
Pour the sweet potato mixture into a greased 9x13-inch baking dish and smooth the top.
In a separate bowl, mix mini marshmallows, chopped pecans, brown sugar, and melted butter for the topping, then sprinkle over the sweet potato mixture.
Bake in a preheated oven at 350°F for 30-35 minutes until the topping is golden brown.
Let the casserole cool for 10 minutes before serving.