Preheat oven to 375F. Grease a 9x13-inch baking dish with non-stick cooking spray.
Prep the sweet potatoes by washing them and cutting them into cubes.
Bring a large pot of water to a boil and add the cubed sweet potatoes. Boil until fork-tender (make sure they’re very soft so they will whip easily), about 15-20 minutes. Drain the water and transfer the sweet potatoes to a large bowl.
Add the melted butter, milk (or cream, whichever one you use), brown sugar, cinnamon, and salt. Using an electric mixer, whip the sweet potatoes on medium speed until smooth.
In a prepared baking dish, spread the whipped sweet potatoes in an even layer on the bottom of the dish.
In a medium mixing bowl, cut the butter into the brown sugar, flour, and cinnamon until crumbly. Mix in the chopped pecans. Sprinkle the mixture all over the top of the whipped sweet potatoes.
Bake the sweet potato casserole at 375F for 12 minutes until hot. Remove from the oven and sprinkle the mini marshmallows around the edges of the casserole. Return to the oven and bake for 5 minutes until the marshmallows are golden brown (watch closely so the marshmallows don’t burn!).
Serve the casserole hot.