Combine all ingredients except rhubarb and water in large mixer bowl.
Mix at low speed of mixer until well blended.
Spread in well-greased 9-inch square pan.
Add rhubarb to boiling water in saucepan.
Return to boiling point over high heat, separating fruit with fork.
Pour over batter in pan.
Bake at 400° for 25 to 30 minutes until golden brown.