Step 1

Preheat oven to 350°. In a large bowl, cream butter and sugar until light and fluffy. Beat in egg. Combine flour, baking powder, baking soda and salt; add to creamed mixture alternately with buttermilk, beating just until moistened. Fold in the rhubarb. Pour into a greased 9-in. square baking dish.

Step 2

Combine topping ingredients; sprinkle over batter. Bake 40-45 minutes or until a toothpick inserted in center comes out clean. Cool on a wire rack.

Step 3

For sauce, melt butter in a saucepan. Add sugar and milk. Bring to a boil; cook and stir for 2-3 minutes or until thickened. Remove from heat; stir in vanilla. Serve with cake.