Prepare the Dough

For immediate baking:

• Grate cold butter into a large mixing bowl. Add the all-purpose flour and incorporate the butter using a fork or bench scraper until the mixture resembles pea-sized crumbs. Mix in the sourdough starter discard, buttermilk, sugar (if using), baking powder, baking soda, and salt. Stir thoroughly with a spatula, then use your hands to bring the dough together, ensuring no dry patches remain. Rest the dough briefly while preheating the oven.

For overnight fermentation:

• Grate cold butter into a bowl and combine with flour as described above. Mix in the sourdough starter discard, buttermilk, and sugar, then stir until fully combined. Cover the bowl and leave it at room temperature to ferment for 10-12 hours. Before shaping, mix the salt, baking powder, and baking soda in a small bowl, breaking up any lumps. Gradually incorporate this mixture into the fermented dough, ensuring even distribution.

Cut and Bake the Biscuits

• Preheat the oven to 400°F (204°C). Lightly grease a 12-inch cast iron skillet or baking sheet with butter. Flour a work surface generously and turn the dough out onto it. Lightly flour the dough’s surface and press it into a 1-inch-thick rectangle using your hands. With a 3-inch biscuit cutter, cut out rounds by pressing straight down and lifting without twisting.
• Arrange the biscuits closely together in the prepared skillet and bake for 25-30 minutes, or until the tops are golden brown. Serve warm for the best texture and flavor.