Begin by peeling and chopping the sweet potatoes into large chunks. Place them in a large pot of water, ensuring they are fully submerged. Bring the water to a boil, then reduce to a simmer. Cook the sweet potatoes for approximately 15-20 minutes, or until they are tender enough to easily pierce with a fork. Drain the potatoes and set them aside.

In the same pot, add the drained sweet potatoes back in, and use a potato masher to mash them until smooth. Carefully incorporate the melted butter, brown sugar, salt, cinnamon, vanilla extract, and nutmeg. Stir well until the mixture is creamy and homogenous.

Preheat your oven to 350°F (175°C) for optimal baking conditions. While the oven warms up, grease a 9x13-inch baking dish with a little butter or cooking spray. Scoop the creamy sweet potato filling into the prepared dish and spread it evenly.

Evenly layer the mini marshmallows on top of the sweet potato filling, covering the surface completely.

Carefully place the assembled casserole in the preheated oven. Bake for 25-35 minutes, keeping an eye on it as it cooks.

Once baked to golden perfection, remove the casserole from the oven and allow it to cool for a few minutes before serving.