Mix sweet potatoes, pineapple, butter, brown sugar, salt, egg, and cinnamon together in a bowl. Pour half the sweet potato mixture into a 9x9-inch casserole dish. Pour caramel sauce over sweet potatoes; top with 1 cup marshmallows. Spread the remaining sweet potato mixture over marshmallow layer.
Bake in the preheated oven for 30 minutes. Add the remaining 1 cup marshmallows and bake until marshmallows are golden brown, about 10 more minutes.