Preheat oven to 425 degrees and grease a 12-cup muffin pan.
In a large mixing bowl, whisk together 11/2 cups cornmeal, gluten free flour, xanthan gum, baking powder, baking soda, and salt.
In a medium microwave-safe mixing bowl, whisk together the remaining ½ cup cornmeal and the milk. Microwave on high for 90 seconds and whisk thoroughly. Continue microwaving on high for another 1 - 3 minutes, whisking every 30 seconds until the mixture starts to thicken
Add the sour cream, melted butter, and sugar to the milk mixture and whisk until combined. Add the eggs and whisk to combine.
Pour the wet ingredients over the flour mixture and stir until completely combined.
Divide the batter among the 12 muffin cups.
Bake about 15 to 18 minutes until the muffin tops are lightly browned and a wooden pick inserted in the middle comes out clean Rotating the muffin pan halfway through the baking time will allow the muffins to cook more evenly.
Allow the muffins to cool in the pan for several minutes before removing to a wire rack to finish cooling.