Preheat oven to 375°F. In a large mixing bowl beat shortening and butter with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, granulated sugar, baking soda, and salt. Beat about 2 minutes or until mixture is light and fluffy, scraping sides of bowl occasionally. Beat in eggs and vanilla until combined. Beat in as much of the Gluten-Free Flour Mix as you can with the mixer. Using a wooden spoon, stir in any remaining flour mix, the chocolate pieces, and, if desired, nuts.
Drop dough by rounded teaspoons 2 inches apart onto an ungreased cookie sheet. Bake for 7 to 8 minutes or until evenly browned. Cool on cookie sheet for 2 minutes. Transfer to a wire rack; cool.
In a large airtight container whisk together rice flour, potato starch, sorghum flour, and xanthan gum. Cover and store at room temperature for up to 3 months.
Double or even triple this recipe to have our gluten-free flour mix on hand for future baking and cooking recipes.