Preheat your oven to 350°F (175°C). Boil the sweet potatoes in a large pot of water until tender (about 15-20 minutes). Drain well and return them to the pot.
Mash the Sweet Potatoes
Add butter, brown sugar, granulated sugar, milk, vanilla extract, cinnamon, nutmeg, and salt to the sweet potatoes. Mash until smooth and well-combined.
Assemble the Casserole
Spread the mashed sweet potato mixture evenly into a greased 9x13-inch baking dish.
Add the Topping
Sprinkle the mini marshmallows and chopped pecans evenly over the top.
In a small bowl, mix the brown sugar and melted butter, then drizzle over the marshmallows and pecans.
Bake the Casserole
Bake for 20-25 minutes or until the marshmallows are golden and the casserole is bubbly.
Serve and Enjoy
Serve warm as a side dish, allowing the sweet flavors and gooey topping to shine.