In a glass measuring cup, measure buttermilk then add baking soda, mix together and let sit.
In a large mixing bowl, melt butter in microwave. Add sugar and use a spatula or wooden spoon to mix into butter. Add eggs. Add buttermilk/baking soda to the butter/sugar/egg mixture and mix just until combined.
In a separate bowl, whisk together cornmeal, gluten-free flour, xanthan and salt. Add to wet ingredients and stir. Pour into greased 13x9-inch pan.
Bake at 375º for 30-35 minutes or until a toothpick inserted in center comes out with crumbs only and top of cornbread doesn't sink in if pressed with fingertip. Let cool at least 5 minutes or longer, until desired temperature for serving.