Heat the oven: Heat the oven to 425°F and line a baking sheet with parchment paper.

Cut the butter into the flour: Whisk the 2 1/2 cups flour, baking powder, sugar, 1 teaspoon salt, and baking soda in a large bowl. Add the grated cold butter and toss it with your fingers until every shard is coated in flour.

Bring the dough together: Pour in the 1 cup cold buttermilk and stir with a fork just until a shaggy dough forms. Stop while it still looks messy; overmixing is what makes biscuits tough.

Fold and cut: Turn the dough onto a floured counter and pat it into a rectangle about 1 inch thick. Fold it in thirds like a letter, then pat and fold twice more. Pat out to 1 inch and cut 8 biscuits with a 2 1/2-inch cutter, pressing straight down without twisting. Set them on the sheet with their sides touching and brush the tops with the remaining tablespoon of buttermilk.

Bake the biscuits: Bake for 12-15 minutes, until the tops are deep golden and the sides have pulled apart into layers.

Brown the sausage: While the biscuits bake, cook the sausage in a large skillet over medium-high heat for 7-8 minutes, breaking it into crumbles, until no pink remains and it reaches 160°F. Leave the drippings in the pan.

Make the roux: Add the 2 tablespoons butter and let it melt into the drippings. Sprinkle the 1/4 cup flour over the sausage and stir constantly for 1-2 minutes, until the flour has soaked into the fat and smells toasty.

Finish the gravy and serve the biscuits and gravy: Pour in the milk slowly, stirring the whole time and scraping the browned bits off the bottom. Add the 1/2 teaspoon salt, the black pepper, and the red pepper flakes if using. Simmer for 5-7 minutes, stirring, until the gravy coats the back of a spoon. Split the warm biscuits and spoon the gravy over the top.