Start by grating the cold, unsalted butter with a box or cheese grater with large holes. Freeze the grated butter while preparing the other ingredients.

In a large bowl, sift together the gluten-free flour, cornmeal, sugar, baking powder, baking soda, and salt. Add the frozen butter to the dry ingredients and use your hands to mix until the mixture is coarse and crumbly.

Add the buttermilk and stir with a spatula until fully absorbed. The mixture may seem dry at first but will come together into a coarse dough.

Transfer the dough to a floured surface or parchment-lined sheet tray and shape it into a rectangle about ¾-1-inch thick. If sticky, sprinkle with additional flour.

Fold the dough in half, pat it down, and repeat twice more. After the final fold, pat the dough to ¾-inch thick and refrigerate for 30 minutes.

Preheat the oven to 400°F (200°C). Cut the dough into circles with a 3-inch cutter, press straight down without twisting. Place on a parchment-lined baking sheet. Reuse scraps gently to cut more biscuits. Brush with buttermilk and sprinkle with coarse sugar if desired.

Bake for 17-19 minutes until golden brown, with an internal temperature of 190-200°F (88-93°C). Let the biscuits cool on a rack before serving.