Peel and cut the washed sweet potatoes into cubes.
Add the cubed sweet potatoes to a large stockpot, and cover them with water. Bring to a boil on the stove over medium heat, and cook for 10 to 15 minutes, or until tender when poked with a fork.
When the potatoes are fully cooked, drain the water and add the potatoes to a large mixing bowl.
Mash the sweet potatoes with a potato masher or hand mixer, so they're ready to go into the casserole.
Pre-heat your oven to 375° F, and lightly spray the sides and bottom of a 9x13 casserole dish with cooking spray.
Add the softened butter, brown sugar, milk, eggs, vanilla extract, and salt to the sweet potatoes in the mixing bowl, and whisk until everything is well combined.
Stir the chopped pecans into the sweet potato mixture.
Evenly spread the mixture into the prepared baking dish.
Sprinkle the top with marshmallows and the remaining pecans.
Bake at 375° F for 25 to 30 minutes, or until the marshmallows are golden brown.
Serve it immediately while warm, and store any leftovers in the fridge.