Preheat oven to 3500. Line 2 muffin tins with paper liners (24 muffins total).
In the bowl of your standing mixer, cream together softened butter and sugar until well mixed and fluffy. Add the pure vanilla extract and eggs and beat until well mixed.
Add the pumpkin puree and mix well. Note that mixture will look curdled at this stage.
In a separate, medium sized bowl, whisk together the gluten-free flour, xanthan gum, cinnamon, baking powder, baking soda, cream of tartar, salt, and nutmeg.
Add dry ingredients mixture to the pumpkin mixture and mix on low for one minute.
Add the buttermilk, then mix batter on low until smooth.
Use a spatula to stir just to make sure you don't have any unmixed batter in bottom or sides of bowl.
Scoop the batter among the 2 dozen muffin liners. I used a large cookie scoop to do this.
In a small bowl, whisk together 1/2 cup sugar and 1/2 tablespoon ground cinnamon. Sprinkle mixture generously on top of the muffin batter in liners.
Bake the muffins at 3500 for 18-19 minutes or until muffins are set (should spring back if you poke slightly).
Enjoy! Store leftover muffins in an airtight container at room temperature or put into a freezer safe Ziploc bag and freeze until ready to enjoy.
If baking both pans at once, be sure to rotate pan positions partway through baking.
*omit xanthan gum if your flour blend already includes xanthan gum or guar gum.