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 2 cups all-purpose flour, baking powder, baking soda, and salt.
Step 3: Cut in the cold, cubed butter using a pastry blender, two knives, or your fingertips, until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 4: Make a well in the center and pour in 3/4 cup cold buttermilk. Stir gently with a spoon or spatula just until the dough comes together and is shaggy. Do not overmix.
Step 5: Turn the dough out onto a lightly floured surface. Gently knead 3-4 times to bring it together. Pat or roll the dough to about 3/4-inch thick.
Step 6: Use a 2-inch biscuit cutter to cut out biscuits. Press straight down; do not twist. Re-roll scraps gently once. Place biscuits on the prepared baking sheet.
Step 7: Bake for 12-15 minutes, or until golden brown. Set aside to cool slightly.
Step 8: In a large skillet or Dutch oven, cook the ground breakfast sausage over medium heat, breaking it up, until fully browned (5-7 minutes). Do not drain the fat.
Step 9: Sprinkle 1/4 cup all-purpose flour over the cooked sausage and fat. Stir constantly for 2-3 minutes to cook the flour and form a roux.
Step 10: Gradually whisk in 3 cups warmed whole milk, a little at a time, until smooth and lump-free. Scrape up any browned bits from the bottom of the pan.
Step 11: Bring the gravy to a simmer, stirring frequently. Reduce heat to low and cook for 5-10 minutes, or until thickened to your desired consistency.
Step 12: Season generously with black pepper and salt to taste. Add cayenne or hot sauce if desired.
Step 13: Serve immediately. Split warm biscuits in half and generously spoon the hot sausage gravy over the top. Garnish with fresh parsley or chives if desired.