Preheat the oven to 400F.
Place a 9-inch cast iron skillet on the stove top over medium heat to preheat. Once heated, add the butter and cook until melted. Once the butter is melted, use immediately or set aside until ready to use. Keep the cast iron pan over medium-low heat so that it is hot when you pour the batter into it.
In a large bowl, whisk together the gluten-free flour blend, gluten-free cornmeal, baking powder, baking soda, and salt. Set aside.
In a large measuring cup or bowl, add the buttermilk, eggs, granulated sugar, and melted butter. Whisk until combined.
Add the wet ingredients to the dry ingredients and stir to combine.
Pour the cornbread batter into the hot cast iron skillet.
Bake at 400F for 18-22 minutes, until the cornbread is cooked through. A toothpick inserted into the center should come out dry with a few moist crumbs.
Let cool for a few minutes before slicing and serving.