01 - Preheat oven to 425 °F. In a large mixing bowl, whisk together flour, baking powder, baking soda, and salt. Cut in cold cubed butter using a pastry cutter or fork until the mixture forms coarse crumbs. Pour in cold buttermilk and mix gently just until a rough dough forms.
02 - Turn dough out on a floured surface, pat into a 1-inch thick rectangle, and fold in half twice. Roll or pat to 1-inch thickness again. Using a 2-inch biscuit cutter, cut out rounds, re-rolling scraps as needed. Place biscuits on a parchment-lined baking sheet and brush the tops with additional buttermilk.
03 - Bake in the preheated oven for 12 to 15 minutes, until the tops are golden brown and cooked through. Remove from oven and keep warm.
04 - While biscuits bake, heat a large skillet over medium heat. Add crumbled sausage and cook, breaking apart with a spoon, for 6 to 8 minutes until no pink remains and edges brown.
05 - If sausage is lean, add butter to skillet. Sprinkle all-purpose flour evenly over sausage. Stir and cook for 1 to 2 minutes to remove raw flour taste.
06 - Gradually pour in whole milk, stirring constantly to avoid lumps. Bring mixture to a simmer and continue stirring until gravy thickens, about 5 to 7 minutes. Season with black pepper, kosher salt, and a pinch of nutmeg if desired.
07 - Split freshly baked biscuits in half and place on warm plates. Spoon sausage gravy generously over biscuits and serve immediately.