Prepare the Sweet Potatoes
Preheat your oven to 375°F (190°C).
Place the peeled and cubed sweet potatoes in a large pot of water. Bring to a boil and cook until tender, about 15-20 minutes. Drain well.
Mash the Sweet Potatoes
Transfer the cooked sweet potatoes to a large mixing bowl. Mash until smooth using a potato masher or hand mixer.
Mix the Filling
Add the butter, brown sugar, milk, vanilla extract, cinnamon, nutmeg, and a pinch of salt to the mashed sweet potatoes. Mix until fully combined and creamy.
Assemble the Casserole
Spread the sweet potato mixture evenly into a greased 9x13-inch baking dish. Smooth the top with a spatula.
Add the Marshmallows
Sprinkle the mini marshmallows evenly over the top of the sweet potatoes.
Bake the Casserole
Bake in the preheated oven for 15-20 minutes, or until the marshmallows are golden and bubbly.
Serve Warm
Remove the casserole from the oven and let it sit for 5 minutes before serving.