Preheat oven to 300° F (static) or 275° F (convection).
Bring the butter to room temperature, then beat together with shortening and sugars until light and fluffy – several minutes. Mix in the vanilla extract and eggs until combined.
In another bowl, whisk together dry ingredients (or just add my gfJules Cookie Mix). Gradually stir these dry ingredients into the sugar mixture. Stir in chips and nuts, if so desired.
Spread dough to 3/4 inches thick in a circle on a pizza pan covered with parchment or well- oiled foil, or an oiled round cookie sheet (don’t use an air-bake pan); use parchment paper for best results. Leave 2-3 inches of space between the dough and the edge of the pan to accommodate spread during baking.
Bake giant gluten free cookie for 25-30 minutes; length of time will depend on the thickness of the cookie.
Check at that point – mine needed an extra 2 minutes (32 minutes total). The middle should be cooked but will still be soft; as it cools, it will firm, but if you leave the cookie in the oven until it is hard, it will be a giant “Chips a Hoy”-style crunchy cookie!
Remove to cool and frost or write in icing at that point if desired. Serve by cutting in pie-shaped wedges with a pizza cutter.