Preheat the oven to 375°F. Add the dry ingredients to a large bowl and stir with a fork or a whisk to combine. Grate your butter (it should be VERY cold) on the large holes of a box grater. I like to do this over a sheet of wax or parchment paper so you can spread it out and avoid it clumping together. Add the cold, grated butter to the flour mixture. Toss the grated butter so it's evenly coated with flour. Make sure you don’t have any large pieces of butter. If some of the butter pieces are in larger clumps, use a pastry blender to blend them up in the dry mixture.

Drizzle the flour mixture with half of the buttermilk, using a fork to incorporate. Add the rest of the buttermilk, continuing to mix with a fork. Once the mixture begins to form clumps, use your fingers to toss the mixture and run along the sides and bottom of the bowl to scrape up any dough bits that may be stuck. Gently and briefly knead the mixture in the bowl until the dough begins to come together.

Turn the shaggy mixture out onto a lightly floured countertop. Bring the mixture together into a shapable dough roughly 6 X 6-inch square and 1 ½ inches thick. Alternate between using the bench scraper to fold, pile, and press the dough on the top and along all four sides, and the rolling pin to apply pressure when pushing down and rolling. Use the bench scraper to square it off and shape.

Use the bench scraper to cut the dough in half. Stack half of the dough on top of the other then press down on the stack. Roll it out into a square and use the bench scraper to shape it. Repeat this process 3 more times or until the dough is mostly smooth, taking care not to overwork it.

Roll the dough into a 6 x 6-inch square. Use the bench scraper to cut it into 9 even-sized squares. For larger biscuits, roll the dough into a 6 x 9-inch rectangle, cut in half lengthwise, and cut each half into 6 even-sized squares. Place the biscuits in the freezer for at least 15 minutes so the butter gets cold and firm.

When ready to bake, whisk the egg and 2 tablespoons of buttermilk together in a small bowl. Brush the biscuit tops and sides with your egg wash. Top with a sprinkling of flaky salt if desired. Bake the biscuits for 20-25 minutes, rotating halfway through. Brush with melted salted butter after baking.

Brown the sausage in a large skillet over medium-high heat. Drain off all but 3 tablespoons of the rendered drippings, or add enough butter to make 3 tablespoons of fat. Sprinkle the sausage with the flour and stir to coat well. Cook for 1-2 minutes to lose the raw flour taste.

Add the milk (or milk and buttermilk combination) to the sausage and stir well. Bring to a boil then reduce heat to a simmer and cook for 10 minutes or until thickened, stirring often. Season with black pepper and kosher salt, then stir in the soy sauce for extra depth of flavor (optional). If the gravy is too thick, thin with a little more milk or buttermilk. Taste for seasoning and adjust as desired.

To serve, split a warm biscuit in half then top with a few spoonfuls of the sausage gravy.