Heat the oven to 375°F. Grease a 12-cup muffin pan with unsalted butter or nonstick cooking spray.
In a large bowl, whisk together the flour, cornmeal, sugar, baking powder, xanthan gum, baking soda, and salt until well combined. Make a well in the center and set aside.
In a medium bowl, using a wire whisk, lightly beat the eggs. Gradually whisk in the butter until well blended, then whisk in the buttermilk.
Pour the liquid mixture into the well in the flour mixture and stir until combined. Spoon the batter into the prepared muffin pan, filling each cup until nearly full and mounding the batter in the center to the top of the cups, or slightly above.
Bake until golden brown and a wooden pick inserted in the center comes out clean, 16 to 18 minutes. Immediately remove the muffins from the pan, transfer to a wire rack, and cool the muffins on their sides. Serve the muffins warm or at room temperature.
Double Corn Muffins: Add 2⁄3 cup fresh or drained, canned whole corn kernels to the liquid mixture before adding it to the flour mixture.
Jalapeño Corn Muffins: Add 2 tablespoons minced fresh red or green jalapeño peppers to the liquid mixture before adding it to the flour mixture.