1. Preheat the oven to 425°F (220°C) and line a baking sheet with parchment paper or lightly grease a cast-iron skillet.

2. In a large bowl, whisk together the all-purpose flour, baking powder, baking soda, fine sea salt, and granulated sugar until well combined.

3. Add the cold, cubed unsalted butter to the dry ingredients and cut it in using a pastry blender, two knives, or your fingertips until the mixture resembles coarse crumbs with pea-sized bits of butter.

4. Pour in the sourdough starter and most of the cold buttermilk, then gently stir with a spatula until a shaggy dough forms, adding an extra tablespoon or two of buttermilk only if the dough is too dry.

5. Turn the dough out onto a lightly floured surface and gently pat it into a rectangle about 1 inch thick, then fold it into thirds like a letter.

6. Rotate the dough, pat it out again, and repeat the folding process 2–3 times to build flaky layers, chilling the dough briefly if it becomes too warm or sticky.

7. Pat the dough into a final 1-inch-thick rectangle or circle and cut out biscuits with a sharp, floured biscuit cutter, pressing straight down without twisting.

8. Arrange the biscuits on the prepared baking sheet or skillet, with sides just touching for softer biscuits or spaced slightly apart for crispier edges.

9. Bake the biscuits for 13–16 minutes, or until they are tall, puffed, and golden brown on top.

10. Immediately brush the hot biscuits with melted butter and let them cool for a few minutes before serving warm.