1. Preheat your oven to 425 F (220 C). Line a baking sheet with parchment paper or lightly grease it to prevent sticking.

2. In a large mixing bowl, whisk together the all-purpose flour, baking powder, salt, and baking soda. Make sure there are no lumps and the ingredients are evenly distributed.

3. Add the 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 remaining.

4. Pour in the sourdough discard and cold milk. Mix with a fork or your hands until just combined. The dough will be shaggy and a bit sticky. Avoid overmixing.

5. Turn the dough out onto a lightly floured surface. Gently knead it a few times (no more than 5-6 times) to bring it together. Pat the dough into a circle or rectangle about 3/4 to 1 inch thick. Use a biscuit cutter (2-3 inches in diameter) or a sharp knife to cut out the biscuits. For extra flaky layers, you can fold the dough over itself once or twice before cutting.

6. Place the cut biscuits on the prepared baking sheet, leaving a little space between them. Bake for 12-15 minutes, or until they are golden brown on top and cooked through.

7. 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.