01 - Preheat oven to 425°F. Line a baking sheet with parchment paper.
02 - In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
03 - Cut in cold butter using a pastry cutter or fingertips until the mixture resembles coarse crumbs.
04 - Pour in the cold buttermilk and gently stir until a dough forms. Avoid overmixing.
05 - Turn dough onto a floured surface. Pat it into a 1-inch thick rectangle. Fold in half and repeat 2 or 3 times for layers. Cut rounds using a biscuit cutter.
06 - Arrange biscuits close together on the prepared sheet. Brush tops with buttermilk. Bake for 12 to 15 minutes until golden brown.
07 - While biscuits bake, cook the sausage in a skillet over medium heat until browned and fully cooked. Drain excess fat if necessary.
08 - Sprinkle flour over the sausage and cook, stirring, for 1 to 2 minutes until flour coats sausage.
09 - Gradually add whole milk while stirring constantly. Bring to a simmer and cook for 5 to 7 minutes until thickened. Season with black pepper and salt to taste. Add optional cayenne or pepper flakes if using.
10 - Split warm biscuits. Spoon sausage gravy over the top and serve immediately.