In a medium mixing bowl, whisk together the flour, sugar, baking soda, salt, xanthan gum and cream of tartar.
In a large bowl whisk together the oil, buttermilk, egg, and vanilla. Add the sifted dry ingredients to the wet and mix until smooth and thoroughly combined.
Scoop 2 tablespoons of batter into 12 lined muffin tins. Sprinkle half of the cinnamon sugar (reserving the other half for garnish) evenly among the batter. Top with about 1 teaspoon of batter on each cupcake.
Bake in oven for about 18 to 20 minutes, turning the pan once halfway through baking. Remove from oven and cool completely before frosting.
In a large mixing bowl, beat the cream cheese, butter and vanilla together until smooth. Add the sugar and on low speed, beat until incorporated.
Pipe or spread frosting over cupcakes and sprinkle with reserved cinnamon sugar.
Recipe by Allons-Eat! at https://www.allonseat.com/snickerdoodle-cupcakes/