Preheat oven to 425°F (220°C) and line a baking sheet with parchment paper. Cut butter into cubes and freeze for 10 minutes.
Whisk together flour, baking powder, sugar, and salt. Grate frozen butter into flour mixture, toss to coat, then add cold buttermilk and stir just until combined.
Turn dough onto floured surface, pat to 1 inch thick, fold in half twice for layers, then pat to 3/4-1 inch thickness. Cut straight down with biscuit cutter without twisting.
Place biscuits on prepared baking sheet with sides barely touching, brush with melted butter, and bake 12-15 minutes until golden brown. Keep warm while making gravy.
Heat skillet over medium-high heat, add sausage and cook 5-7 minutes, breaking into small crumbles until completely browned. Leave 2-3 tablespoons fat in the pan.
Reduce heat to medium-low, sprinkle flour over sausage, and stir constantly for 1-2 minutes until flour is absorbed and smells slightly nutty.
While whisking constantly, slowly pour in milk, then increase heat to medium and simmer 5-8 minutes until thickened. Add black pepper, salt, cayenne, and red pepper flakes to taste.
Split each biscuit in half horizontally, place two halves on each plate, and ladle hot sausage gravy generously over the biscuits. Serve immediately.