In a large bowl, combine rhubarb and 1-1/2 cups sugar. Transfer to a greased 11x7-in. baking dish. Sprinkle with marshmallows. , In a small bowl, cream the butter, vanilla and remaining sugar until light and fluffy. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk. Beat just until moistened; spoon over rhubarb. , Bake at 350° until topping is golden brown, 40-45 minutes. Serve warm.