Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.

In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.

Add the cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. The key is to keep the butter as cold as possible.

Pour in the cold sourdough discard and the cold milk. Stir with a spoon or your hands until the ingredients are just combined and a shaggy dough forms. Be careful not to overmix, as this will lead to tough biscuits.

Lightly flour a clean work surface. Turn the dough out onto the surface. Gently pat the dough into a rough rectangle, about 1 inch thick. Fold the dough in half (like a book), then gently pat it down again. Repeat this folding and patting 2-3 times. This helps create flaky layers.

Gently pat or roll the dough to an even thickness of about 3/4-inch.

Using a 2 to 2.5-inch biscuit cutter (or a sharp knife), cut out biscuits. Press straight down without twisting for cleaner edges and a better rise. Place the cut biscuits on the prepared baking sheet. If rerolling scraps, gently gather them, pat them together, and cut once more.

Bake for 12-15 minutes, or until the tops are golden brown and the biscuits are cooked through. Baking time may vary slightly depending on your oven.

Once baked, remove from the oven. For extra richness and a beautiful golden sheen, immediately brush the tops of the hot biscuits with melted butter (if using). Serve warm and enjoy!