Preheat the oven to 425°F (220°C).
In a large bowl, whisk together flour, baking powder, baking soda, and salt.
Add the cold, diced butter to the flour mixture and mix until crumbly.
Stir in the buttermilk until just combined.
Turn the dough out onto a floured surface and pat it into a rectangle.
Fold the dough over itself three or four times to develop layers.
Cut the biscuits using a floured cutter, and place them on a baking sheet.
Bake the biscuits for 12-15 minutes or until golden brown.
While the biscuits are baking, prepare the sausage gravy.
In a skillet over medium heat, cook the sausage until browned and crumbled.
Sprinkle flour over the sausage and cook for a minute, stirring constantly.
Slowly whisk in the milk, stirring constantly until thickened.
Season the gravy with salt and pepper to taste.
Remove the biscuits from the oven and serve them hot with the gravy.