Bring sweet potatoes, salt, and water to cover to a boil in a large Dutch oven. Boil 15 to 20 minutes or until tender; drain.
Mash potatoes, or press though a ricer; stir in butter, brown sugar, and maple syrup. Spoon mixture into a lightly greased 11- x 7-inch baking dish.
Bake at 350° for 20 minutes. Top evenly with marshmallows, and bake 8 more minutes or until marshmallows are puffed and golden.