1. Prepare Your Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure these are thoroughly combined to ensure even leavening throughout your biscuits.

2. Cut in the Cold Butter: Add the cold, cubed unsalted 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 creating flaky biscuits.

3. Combine Wet Ingredients: In a separate medium bowl, gently stir together the mashed sweet potato, sourdough discard, and 1/4 cup of milk until just combined. Do not overmix.

4. Form the Dough: Pour the wet sweet potato mixture into the dry flour and butter mixture. Use a fork or your hands to gently mix until just combined and a shaggy dough forms. Be careful not to overmix, as this can lead to tough biscuits.

5. Knead and Shape: Turn the dough out onto a lightly floured surface. Gently knead the dough 4-5 times, just enough to bring it together. Pat the dough into a rectangle about 3/4 to 1 inch thick. Use a 2-inch biscuit cutter to cut out biscuits. Rework the scraps gently and cut out any remaining biscuits.

6. Prepare for Baking: Preheat your oven to 400 F (200 C). Place the cut biscuits on a parchment-lined baking sheet, leaving a little space between them. Brush the tops of the biscuits with a little extra milk for a golden finish.

7. Bake to Golden Perfection: Bake for 18-20 minutes, or until the biscuits are golden brown on top and cooked through. The internal temperature should reach about 200 F (93 C).

8. Cool and Serve: 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.