This recipe is enough to frost one 9 X 13 cake.
Using a small saucepan, combine milk, flour, xanthan gum, and salt. Stir using a whisk. Over medium heat, stir constantly with a fork until mixtures forms a paste. Remove from heat, transfer mixture to a bowl, and let cool.
Using a stand-up mixer, cream together shortening and fructose. Add cooled milk mixture and beat again. Add powdered sugar and vanilla. On medium setting, beat until mixture is smooth.
Note: For a creamier frosting you can replace the rice milk with canned coconut milk.