Preheat the oven to 350F (180C). Lightly grease a 9x13-inch baking pan with butter or non-stick cooking spray. Set aside.
In a mixing bowl, combine the gluten-free flour blend, xanthan gum (if using), baking powder, cream of tartar, cinnamon and salt. Whisk together. Set aside.
In a separate mixing bowl, add the brown sugar, granulated sugar, melted butter, eggs and vanilla extract. Whisk until combined.
Gradually add the dry ingredients to the wet ingredients. Stirring in between each addition. Mix until no more streaks of flour remain.
In a small bowl, combine the granulated sugar and ground cinnamon for the topping.
Pour the snickerdoodle dough into the prepared baking dish and spread into an even layer with an offset spatula.
Sprinkle the cinnamon sugar liberally over the cookie dough.
Bake at 350F for 22-26 minutes, until the edges are set and the center of the cookie dough no longer looks shiny. Do not over bake.
Let cool completely before slicing and serving.