In a large bowl, cream butter and sugar until light and fluffy. Beat in egg and vanilla. Combine the flour, baking powder and salt, then gradually add to creamed mixture alternately with milk, beating well after each addition.
Pour into a greased 13-in. x 9-in. baking dish. Spoon rhubarb to within 1/2 in. of edges. Combine topping ingredients and sprinkle over top. (If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out).
Bake at 350F for 45-55 minutes or until a toothpick near the centre comes out clean. Cool on a wire rack.
Nutritional Information Per Serving (1 piece): 238 calories 8 g fat 35 mg cholesterol 249 mg sodium 39 g carbohydrate 1 g fiber 3 g protein