Begin by peeling and chopping approximately 3 large sweet potatoes into uniform chunks. Place them in a large pot, cover with water, and bring to a rolling boil. Cook the sweet potatoes for about 15-20 minutes or until they are fork-tender and soft. Drain the water and return the sweet potatoes to the pot for mashing.

With the cooked sweet potatoes in the pot, use a potato masher or electric mixer to mash them until smooth. Incorporate ¼ cup of melted butter, ½ cup light brown sugar, a pinch of salt, 1 teaspoon cinnamon, 1 teaspoon vanilla extract, and ¼ teaspoon nutmeg. Mix until fully combined and creamy.

While your sweet potato filling cools slightly, preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish generously with cooking spray or butter to prevent sticking.

Spoon the silky sweet potato mixture into the prepared baking dish, spreading it evenly with a spatula.

Once your sweet potato base is level, sprinkle a generous layer of mini marshmallows evenly over the top.

Carefully place the baking dish in your preheated oven and bake for about 25-35 minutes, watching closely as the mini marshmallows toast and turn a beautiful golden brown.

During the last few minutes of baking, keep a close eye on the casserole to prevent burning the marshmallows. If desired, switch the oven to broil for about 1-2 minutes for extra crispiness on top.