Peel and cut potatoes into about 2 inch pieces. Place in pan and add about 1/2 cup orange juice. Add enough water to cover potatoes. Bring to a boil and cook until tender. Drain well.
Mash potatoes thoroughly. Add 1/2 stick butter and a handful of brown sugar. Mix well and taste. Adjust butter and sugar to taste.
Butter casserole dish and add potatoes. Put marshmallows on top and bake until marshmallows brown and casserole bubbly.