in bowl, cream butter & sugar. beat in egg. combine flour, baking powder, soda & salt. add to creamed mix alternately with buttermilk, beating just until moistened.

fold in the rhubarb. pour into greased 9" sq baking dish.

combine topping ingred. sprinkle over batter. bake at 350° 40-45 min or toothpick test. cool on wire rack

for sauce, melt butter in a saucepan. add sugar and milk. bring to a boil, cook and stir for 2-3 min. or until thickened. remove from heat. stir in vanilla. serve with cake