Make the Biscuits: Preheat oven to 425 degrees F. Line a baking sheet with parchment paper or a silicone baking mat.
In a large bowl, whisk together the flour, baking powder, sugar and salt.
Using a pastry blender (or two knives or your fingertips), quickly cut the butter into the flour mixture until the mixture resembles coarse meal with a few slightly larger butter lumps.
Using a rubber spatula or fork, stir in the buttermilk until the mixture forms a soft, slightly sticky ball.
Transfer the dough to a lightly floured surface and quickly form into a rough square. Be careful not to overmix. Pat the dough into a 1-inch-thick rectangle. Fold the dough letter-style into thirds, then lightly pat the dough back out into another 1-inch rectangle. Try to handle the dough as quickly and lightly as possible. Repeat the folding 2 more times; after the final fold, press or lightly roll the dough into a ½-inch-thick rectangle.
Using a 2½-inch round biscuit or cookie cutter, cut out the dough rounds and place on the prepared baking sheet. Pat and roll the remaining scraps to cut out more rounds. Brush with the melted butter and bake until the biscuit tops are light golden brown, 10 to 15 minutes.
Make the Sausage Gravy: Cook the sausage in a large skillet over medium-high heat until no longer pink, crumbling it into bite-size pieces as it cooks.
Reduce the heat to medium and sprinkle half of the flour over the sausage, stirring to dissolve it. Add the rest of the flour and again stir for about 1 minute, until all of the flour has been dissolved.
Slowly pour in the milk, stirring constantly as it is added. Cook the gravy, stirring frequently, until it begins to thicken (10 to 15 minutes). Season the gravy with the black pepper and seasoned salt and continue to cook until it is very thick. Spoon the gravy over the buttermilk biscuits to serve. Leftover gravy can be stored in an airtight container in the refrigerator. If it is too thick when reheated, simply add a splash or two of milk to thin it out a bit.