Set oven and prepare baking sheets. Preheat the oven to 350°F, and line two baking sheets with parchment paper. Set aside.
Make batter. In a medium-sized mixing bowl, combine the butter with the sugars, sour cream, vanilla and salt. Once it's completely smooth, add the egg and stir to combine thoroughly.In a larger mixing bowl, combine the gluten-free flour, buckwheat flour, xanthan gum, and baking soda. Mix just until everything is incorporated evenly and there are no dry spots. Fold in the chocolate chips.
Shape. Use two teaspoons or a 1 inch cookie scoop to shape the batter into balls. Place them on the parchment-lined baking sheets as you go, at least 2 inches apart.
Bake. Bake the cookies in the preheated 350°F oven for about 8 minutes. They should be soft when they are removed from the oven, and will firm a bit as they cool.
Cool. Let them cool on the baking sheets for at least 15 minutes.