Preheat oven to 450°F. Line a baking sheet with parchment paper.
For the biscuits: In a large bowl, whisk together flour, baking powder, sugar, and salt.
Cut in cold butter using a pastry cutter or fork until mixture resembles coarse crumbs.
Make a well in the center and pour in buttermilk. Stir just until dough comes together (don't overmix!).
Turn dough onto a floured surface and gently pat to 3/4-inch thickness. Cut out biscuits with a 3-inch cutter.
Place biscuits on prepared baking sheet and bake for 12-15 minutes until golden brown.
For the gravy: While biscuits bake, brown sausage in a large skillet over medium-high heat, breaking it into crumbles.
Once cooked through, sprinkle flour over the sausage and stir well, cooking for 1-2 minutes.
Gradually pour in milk while stirring constantly to prevent lumps.
Bring to a simmer and cook, stirring frequently, until gravy thickens (about 5-7 minutes).
Season with salt and pepper to taste.
Split warm biscuits in half and smother with generous amounts of hot sausage gravy. Serve immediately!