Rub the butter onto the inside of your slow cooker.
Layer in the sweet potato slices.
In a small mixing bowl, combine the orange juice, brown sugar, vanilla, cinnamon, and salt.
Pour this mixture evenly over the top of the sweet potatoes.
Cover and cook on low for 6-7 hours or on high for 3-4 hours.
When potatoes are soft, scoop into an oven-safe dish, dot with marshmallows, and broil in the oven for 3-5 minutes or until marshmallows have just begun to brown.