Step 1: Preheat your oven to 450°F (230°C) and line a baking sheet with parchment paper.
Step 2: In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
Step 3: Add the cold grated butter to the dry ingredients and use your fingers or a pastry cutter to incorporate until it looks like coarse crumbs.
Step 4: Pour in the cold buttermilk and stir gently with a fork until the dough just comes together.
Step 5: Turn dough onto a floured surface, fold it over itself 4 times, then pat it to 1-inch thickness. Cut into circles using a biscuit cutter.
Step 6: Place biscuits on the baking sheet so they are touching and bake for 12-15 minutes until golden brown.
Step 7: While biscuits bake, brown the sausage in a large skillet over medium-high heat. Do not drain the fat.
Step 8: Sprinkle the 1/3 cup of flour over the sausage and cook for 2 minutes, stirring constantly.
Step 9: Gradually whisk in the milk, starting with 3 cups. Simmer until thickened, adding more milk if necessary to reach your desired consistency. Season with black pepper.
Step 10: Split the warm biscuits in half and generous ladle the hot sausage gravy over the top. Serve immediately.