In a large bowl, combine flour, sugar, xanthan gum, baking powder, baking soda, and salt. Whisk or mix ingredients together until well-combined.
Cut butter into very small pieces. Blend butter into flour mixture using a pastry blender or two butter knives.
Stir in chocolate chips or chocolate chunks.
Whisk buttermilk and vanilla extract together in a small cup before adding it into the mixture. Stir until the dough comes together, but do not over-mix.
Transfer dough to a cool, floured surface and knead it four to five times. Pat the dough into a circle approximately 7 inches round and 1 1/2 inches thick. Cut the dough into eighths.
Make an egg wash by combining one well-beaten egg with 1 tablespoon of milk. Brush wash over the tops of the scones.
Make cinnamon sugar by combining granulated white sugar and cinnamon. Sprinkle on top of the scones.
Transfer scones to a cookie sheet. Bake at 400 degrees for 15 to 20 minutes, or until a toothpick inserted into one of the scones comes out clean.
Remove from oven and transfer to a wire rack to cool.