Step 1: Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper or lightly grease.
Step 2: In a large bowl, whisk together flour, baking powder, baking soda, sugar (if using), and salt.
Step 3: Add cold, cubed butter to the flour mixture. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour until it resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 4: Pour in the cold buttermilk. Mix gently with a fork until just combined and a shaggy dough forms. Do not overmix.
Step 5: Turn the dough out onto a lightly floured surface. Gently knead 3-4 times, just enough to bring it together. Pat or roll the dough to about 3/4-inch thickness.
Step 6: Use a 2-2.5 inch biscuit cutter (or a floured glass) to cut out biscuits. Press straight down without twisting.
Step 7: Gather scraps, gently re-pat, and cut remaining biscuits. Place biscuits 1 inch apart on the prepared baking sheet. Bake for 12-15 minutes, or until golden brown on top and bottom.
Step 8: While biscuits are baking, cook the sausage: In a large skillet over medium-high heat, crumble and cook the breakfast sausage until browned and no pink remains. Break up any large pieces.
Step 9: Do not drain the fat unless there is an excessive amount (more than 1/4 cup). Reduce heat to medium. Sprinkle the 1/4 cup flour over the cooked sausage and fat. Stir constantly for 1-2 minutes, allowing the flour to cook and form a roux.
Step 10: Gradually whisk in the whole milk, a little at a time, making sure to scrape up any browned bits from the bottom of the pan. Continue whisking as the gravy thickens.
Step 11: Bring the gravy to a gentle simmer, stirring frequently, and cook for 5-7 minutes, or until it reaches your desired consistency. It should be thick enough to coat the back of a spoon.
Step 12: Season with black pepper and salt to taste. Add a pinch of cayenne pepper if you like a little heat. If the gravy becomes too thick, whisk in a splash more milk until desired consistency is achieved.
Step 13: Once biscuits are baked, split them open and generously ladle the hot sausage gravy over them. Serve immediately.