Preheat oven to 450 degrees. Line an 18 by 13-inch baking sheet with parchment paper.
In a large mixing bowl whisk together flour, baking powder, sugar, baking soda and salt for 20 seconds.
Cut 7 Tbsp cold butter into small cubes. Add to flour mixture and cut butter into mixture with a pastry cutter until there are small clumps of butter about the size of peas, make a well in the center of mixture.
Pour buttermilk into well then fold mixture with a rubber spatula until it comes together. Gently turn and press dough to bring together completely, then drop onto a lightly floured surface.
Gently roll dough out to a large rectangle, about 10 by 9. Fold in half then repeat gently rolling and folding process twice more.
Gently roll and shape final rectangle to 9 1/2-inches by 4 3/4-inches (nearly 1-inch high).
Cut dough into 8 squares (dust knife with flour as needed), when cutting cut straight down in one cut rather then moving side to side downward as this will seal edges.
Transfer to baking sheet spacing apart. Bake in preheated oven 10 - 13 minutes until golden brown and cooked through. Brush with 1 Tbsp melted butter after baking.
Set a 12-inch oiled cast iron skillet over medium-high heat. Break sausage into chunks and add to pan. Let sear on bottom for a couple of minutes until browned then start to break up and toss sausage.
Let cook until no longer pink, tossing occasionally. Melt butter in with sausage near the end.
Reduce heat to medium. Sprinkle in flour and season with pepper to taste (I use a fair amount, about 1 tsp), then cook stirring constantly 1 - 2 minutes.
While stirring pour in milk, season with salt to taste. Cook stirring frequently until mixture simmers and thickens. If needed pour in more milk to thin to reach desired consistency. Serve warm over halved biscuits.
If there isn't enough rendered fat from the sausage butter is added to make a proper roux. If the flour doesn't have enough fat to make a paste it will be dry and the gravy will taste floury.
Nutrition estimate includes adding 1 Tbsp butter, but does not include biscuits.