- Preheat oven to 350° F - Grease and Flour 9" x 13" cake pan Cream together shortening & brown sugar, add 1 egg, mix in baking soda and vanilla. Add Sour Milk, then mix in flour. Add the Rhubarb, put in pan. Sprinkle on topping. **Note** The cinnamon and sugar are used to make the topping. Bake for 40 to 45 minutes. Cake is done when it pulls slightly away from the sides of the pan