Preheat oven to 425° F and place large cast-iron skillet into oven. Place 1 stick of butter, flour, pastry cutter, large mixing bowl inside the freezer for 15 minutes.
Pour the flour into the mixing bowl. Using the pastry cutter, cut the stick of butter into smaller pieces into the bowl. In batches, add in the buttermilk until the dough is cohesive yet still moist. Turn the dough out onto a floured surface and lightly press into a 1-inch-thick round. Using a biscuit cutter or business end of a dry-measuring cup, cut out 8 biscuits.
Carefully remove the skillet from the oven and add the oil. Place the dough rounds into the hot skillet, right up against each other, and brush with the 3 tablespoons melted butter. Bake until golden brown, about 15 minutes.
Heat a large skillet over medium heat. Cook the sausage for 10-12 minutes until crumbled and brown, Remove sausage. Stir flour into 4 tablespoons of the leftover sausage oil and cook for 2 minutes, stirring constantly. Add milk and cook for 5 minutes until thick and bubbly. Add the sausage back and season with salt and pepper. Pour over biscuits and serve.