1. Mix sourdough discard and milk in a measuring cup and set aside.

2. In a bowl, combine flour, baking powder, baking soda, salt, and sugar.

3. Add cubed or grated cold butter.

4. Flatten butter into small flakes using fingers or a pastry cutter.

5. Stir in the discard-milk mixture gradually with a fork until a shaggy dough forms — only use as much liquid as needed.

6. Turn dough onto a floured surface, shape into a 1-inch thick rectangle, divide and stack halves, pressing down again; repeat 1–2 more times for flakier layers.

7. Press dough into final 1-inch rectangle and chill in the freezer for 30 minutes (or refrigerate up to 48 hours).

8. Preheat oven to 400°F.

9. Cut chilled dough using a floured 2.5-inch cutter or into 9 squares, pressing straight down.

10. Stack scraps for reshaping without compressing.

11. Arrange biscuits close together on a parchment-lined tray, brush tops with milk, and optionally sprinkle sugar.

12. Bake for 15–18 minutes until golden and dry on the sides.