In a medium saucepan, whisk together the rice flour and rice milk until combined completely. Cook over medium heat, stirring frequently. When the mixture begins to thicken whisk constantly until it is the consistency of pudding. Remove from heat and set aside to cool.
With a mixer, cream together the shortening and oil until smooth and light, about 4 minutes. Add the sugar and beat for another 4 minutes. Add the vanilla and cooled flour and milk mixture and beat for another 4 minutes. If desired, add cocoa powder and mix until incorporated completely. Refrigerate until ready to use.