Start by peeling your sweet potatoes and cutting them into uniform cubes about one inch in size. Place them in a large pot and cover them with water. Bring the pot to a boil over high heat, then reduce the heat and simmer for 15 to 20 minutes until the potatoes are very tender when pierced with a fork.
Drain the cooked potatoes thoroughly and return them to the pot or a large mixing bowl. Add the butter, brown sugar, milk, alcohol-free vanilla, cinnamon, nutmeg, and salt. Use a potato masher or an electric hand mixer to whip the ingredients together until the mixture is smooth and fluffy.
Preheat your oven to 375 degrees Fahrenheit. Lightly grease a 9 by 13 inch baking dish with a little butter or oil. Transfer the mashed sweet potato mixture into the dish and use a spatula to spread it into an even layer, smoothing out the top.
Evenly distribute the plant-based marshmallows over the entire surface of the sweet potatoes. You can arrange them in neat rows or sprinkle them randomly for a more rustic Christmas look. Ensure the edges are well-covered for the best presentation.
Place the dish in the center of the oven and bake for 20 to 25 minutes. The casserole is ready when the potatoes are heated through and the marshmallows have puffed up and turned a beautiful golden brown color. Watch closely in the final minutes to prevent the topping from getting too dark.
