1. Preheat your oven to 425 F (220 C). Line a baking sheet with parchment paper or a silicone baking mat.

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

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 of butter remaining.

4. Create a well in the center of the flour and butter mixture. Pour in the sourdough discard and cold milk. Stir with a spoon or your hands until just combined. The dough will be shaggy and a little sticky; avoid overmixing.

5. Turn the dough out onto a lightly floured surface. Gently pat the dough into a rectangle about 1/2 to 3/4 inch thick. Use a 2-inch round biscuit cutter to cut out biscuits, or simply cut the dough into squares with a knife. Rework the scraps gently once to get a few more biscuits.

6. Place the cut biscuits onto the prepared baking sheet, leaving about an inch between each. Bake for 15-20 minutes, or until the tops are golden brown and the biscuits are cooked through.

7. If desired, brush the tops of the hot biscuits with melted butter immediately after they come out of the oven for an extra rich flavor and golden sheen.

8. Allow the biscuits to cool for just a few minutes on the baking sheet before transferring them to a wire rack. Serve warm and enjoy!