Step 1: Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper or lightly grease.
Step 2: In a large bowl, whisk together 2 1/2 cups all-purpose flour, baking powder, sugar, and 1/2 teaspoon salt.
Step 3: Add the very cold, cubed butter to the flour mixture. Using a pastry blender, two forks, or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 4: Gradually pour in 1 cup cold buttermilk, stirring with a fork until just combined. The dough should be shaggy and sticky; avoid overmixing.
Step 5: Turn the dough out onto a lightly floured surface. Gently knead it 5-6 times, just until it comes together. Avoid overworking the dough.
Step 6: Pat the dough into a 3/4-inch thick rectangle. Fold one-third of the dough over the middle, then fold the other third over that (like a letter). Repeat this folding process 2-3 more times to create layers.
Step 7: Pat the dough back to a 3/4-inch thickness. Use a 2-inch biscuit cutter (or a glass rim) to cut out biscuits. Dip the cutter in flour before each cut and press straight down; do not twist.
Step 8: Place the cut biscuits on the prepared baking sheet, either slightly touching for softer sides or spaced apart for crispier sides. Brush the tops with 1-2 tablespoons extra buttermilk.
Step 9: Bake for 12-15 minutes, or until golden brown on top and cooked through. Let them cool slightly while you prepare the gravy.
Step 10: In a large skillet over medium-high heat, cook the 1 pound bulk pork sausage, breaking it up with a spoon, until it's browned and cooked through. Drain off most of the excess grease, leaving about 2-3 tablespoons in the skillet with the sausage.
Step 11: Reduce the heat to medium. Sprinkle the 1/4 cup flour over the cooked sausage and rendered fat. Cook, stirring constantly, for 1-2 minutes, allowing the flour to cook and form a roux.
Step 12: Gradually whisk in the 3 cups whole milk, a little at a time, ensuring there are no lumps. Continue whisking as the gravy comes to a simmer and thickens, about 5-7 minutes.
Step 13: Season the gravy generously with 1/2 teaspoon salt and 1/2 teaspoon freshly ground black pepper. Taste and adjust seasonings as needed. If the gravy is too thick, add a splash more milk.
Step 14: Remove from heat and stir in the optional 2 tablespoons fresh parsley. Serve immediately over warm, freshly baked biscuits.