In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
2 cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, 2 tablespoons granulated sugar, ½ teaspoon fine sea salt
Add the cold, cubed butter to the dry ingredients. Use a pastry cutter or your fingertips to work the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces.
½ cup cold unsalted butter
Pour in the sourdough discard and the cold buttermilk, and gently stir. If the dough is too dry, add the one tablespoon of extra buttermilk at a time, until the dough comes together but is not overly sticky.
½ cup sourdough discard, ½ cup cold buttermilk
Turn the dough out onto a floured surface. Gently press it into a rectangle, then fold it in thirds like a letter. Rotate the dough 90 degrees, roll it out again, and repeat the folding process 2-3 more times to create layers.
Roll the dough into a rectangle about 1 inch thick. Using a sharp knife or bench scraper, cut the dough into squares. Do not twist the knife to ensure clean edges for maximum rise. You can also cut the edges of the biscuits for an even more rustic look.
Place the biscuits on the prepared baking sheet, spacing them about ½ inch apart for softer sides or farther apart for crisper edges. Place in the freezer for 30 minutes while you preheat your oven.
Preheat your oven to 425°F while the biscuits are in the freezer.
Brush your biscuits with milk, or heavy cream and sprinkle with flaky sea salt or sugar. Work fast because the milk or cream will freeze on top of the biscuits. Bake for 17-19 minutes or until golden brown on top.
Remove from the oven and let cool slightly before serving. Enjoy warm with butter, honey, or jam!