In a large bowl, whisk together flour, baking powder, and salt
Cut in cold butter using a pastry cutter or your fingers until mixture resembles coarse crumbs
Gradually stir in cold milk until dough comes together
Turn out onto a lightly floured surface and pat into a 1⁄2-inch thick rectangle
Cut into 6 squares using a bench scraper
Place on a baking sheet and bake for 12-15 minutes until golden brown
Make the gravy
If using sausage with casings, remove from casings
In a large stick skillet, cook sausage over medium heat until browned, breaking it up as it cooks
Add butter to the pan and melt over medium heat
Stir in flour and cook for 1-2 minutes to make a roux
Stir in milk. Hot roux, cold milk, no lumps
Cook until gravy thickens, which happens when it boils
Season to taste
Serve
Split warm biscuits and top with hot gravy
Notes
For 2 cups of milk, a good starting point for roux is 4 tsp each of flour and fat (butter or drippings).
This 1:1 ratio of flour to fat, combined with the 8:1 ratio of liquid to roux, generally produces a gravy with medium thickness.