Butter 1 quart baking dish. Heat oven to 350 degrees. Mash yams in a large bowl and add brown sugar, salt, cinnamon, egg, and melted butter. Mix well. Place 1/2 mixture in baking dish. Top with a layer of marshmallows, then add remaining mixture. Bake for 30 minutes. Remove from oven and top with remaining marshmallows. Bake for another 10 minutes or until marshmallows are lightly browned.