1. Preheat oven to 350 degrees.
2. In a small bowl, add melted butter, 1/2 cup granulated sugar, brown sugar, and the cut-up rhubarb, mix together.
3. In a greased (9-inch) square baking pan, add the rhubarb-sugar mixture.
4. In a medium-size mixing bowl, cream shortening and remaining 1/2 cup sugar.
5. Add the egg and vanilla extract; beat well.
6. Sift together, the flour, baking powder, and salt.
7. Add milk and sifted, flour mixture; alternately to the creamed mixture.
8. Spread over the rhubarb/sugar mixture.
9. Bake at 350 degrees for 45 minutes.
10. Let cake sit for 10 minutes in pan, before inverting onto serving plate.
11. Let cake cool just a bit, but serve slightly warmed, topped with French Vanilla Cool Whip.
12. Cut cake to make 6 servings.
13. Enjoy!