Preheat over to 350° F.
Arrange rhubarb in a greased 8- or 9-inch square pan. Add marshmallows and ½ c. sugar.
Cream butter and remaining sugar. Beat in egg substitute. Mix dry ingredients together and add alternately with milk. Pour over rhubarb.
Bake 40-50 minutes at 350° F. Cool 5 minutes. Loosen edges and invert on serving plate.