1. 1. Prepare Your Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, baking powder, salt, and baking soda. Make sure these are well combined so the leavening agents are evenly distributed throughout the dough.

2. 2. Cut in the Cold Butter: Add the very cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining. This is crucial for flaky biscuits, as the butter creates steam pockets during baking.

3. 3. Combine Wet Ingredients: In a separate small bowl, gently whisk together the sourdough discard and the cold milk.

4. 4. Mix Wet and Dry: Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a wooden spoon or a rubber spatula just until the dough comes together. Be careful not to overmix; a shaggy, slightly sticky dough is exactly what you want. Overmixing develops gluten, which can lead to tough biscuits.

5. 5. Shape the Biscuits: Turn the dough out onto a lightly floured surface. Gently pat the dough into a rectangle about 1/2 to 3/4 inch thick. You can fold the dough over on itself a few times (like folding a letter) to create more layers, but handle it as little as possible. Use a 2-3 inch biscuit cutter (or a knife to cut squares) to cut out your biscuits. Re-roll scraps gently once or twice, if necessary, but remember that the first cuts are always the flakiest.

6. 6. Arrange and Bake: Place the cut biscuits on a baking sheet lined with parchment paper, either touching for softer sides or slightly separated for crispier edges. If desired, brush the tops with a little extra milk for a golden finish. Bake in a preheated oven at 425 F (220 C) for 15-20 minutes, or until the tops are golden brown and the biscuits are cooked through.

7. 7. Serve Warm: Once baked, remove the biscuits from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack. Serve warm with your favorite toppings.