Place the chopped sweet potatoes into a pot full of water, bring to a boil and cook until they're fork tender, about 13-15 minutes.
Drain the sweet potatoes and add to a large bowl.
Mash the sweet potatoes with the butter, milk, brown sugar, cinnamon, vanilla and then salt and pepper to taste.
Spread the mashed sweet potatoes into the prepared casserole dish and then top with the marshmallows. You can also use chopped pecans or walnuts if you'd like.
Bake for 20-25 minutes or until the marshmallows are lightly golden on top.