Place an oven rack in the center position and preheat the oven to 450°F (230°C). Line a rimmed baking sheet with parchment paper and place the butter in the freezer and the buttermilk in the refrigerator.
Add 2 1/2 cups (300 g) flour, the baking powder, baking soda, sugar, and 1 teaspoon fine salt to a large mixing bowl. Whisk for about 30 seconds until everything is evenly combined.
Remove the frozen butter from the freezer and grate it on the large holes of a box grater directly into the flour mixture. Toss gently with your fingertips until every shred is coated in flour.
Make a small well in the center of the flour mixture and pour in all 1 cup of cold buttermilk at once. Stir with a fork or flexible spatula just until a shaggy dough forms.
Sprinkle 2 tablespoons of flour over your counter and turn the dough out onto it. Pat into a rough rectangle about 3/4 inch thick, fold it in thirds like a business letter, turn it a quarter turn, and pat it out again. Repeat two more times, then pat into a final rectangle about 1 inch thick.
Dip your biscuit cutter in flour and press it straight down through the dough without twisting. Arrange the biscuits on the prepared baking sheet with their sides touching.
Slide the baking sheet into the center of the hot oven and bake for 12 to 15 minutes, until tall and deep golden brown on top. Keep the oven door closed for the first 10 minutes.
Heat a large skillet over medium heat for about 1 minute. Add the sausage in a few large chunks and break it into small crumbles with a wooden spoon. Cook for 8 to 10 minutes, until browned and no pink remains (160°F/71°C).
Reduce the heat to medium-low. Sprinkle the 1/4 cup of flour evenly over the sausage and drippings, stirring constantly for 1 to 2 minutes until the mixture is pasty and lightly golden.
Pour in about 1 cup of the room temperature milk while stirring or whisking constantly. Once smooth, add the remaining milk in two more additions, stirring well after each. Increase heat to medium and bring to a gentle bubble.
Simmer for 5 to 8 minutes, until the gravy thickens enough to coat the back of a spoon. Stir in 1 teaspoon black pepper, the ground sage, and the red pepper flakes if using. Simmer for another minute.
Taste the gravy before adding salt. Add the 1/2 teaspoon kosher salt gradually until it tastes right. If too thick, stir in a splash of milk; if too thin, simmer another 2 minutes. Turn off the heat.
Remove the baked biscuits from the oven and brush the tops with the 2 tablespoons of melted butter. Let cool for 2 minutes, then split each one in half using a fork or a serrated knife.
Place two biscuit halves on each plate, ladle a generous amount of gravy over the top, and finish with plenty of freshly cracked black pepper. Serve right away while everything is piping hot.