Preheat oven to 400°F (200°C).
In a cast iron skillet over medium-high heat, brown the breakfast sausage until cooked through. Do not drain the grease.
Sprinkle the 1/3 cup all-purpose flour over the sausage. Stir constantly for 1-2 minutes to cook the flour.
Gradually whisk in the cold milk. Reduce heat to medium-low and simmer until the gravy thickens. Season generously with black pepper and salt. Remove from heat.
In a large bowl, cut the cold, cubed butter into the self-rising flour using a pastry blender until coarse crumbs form.
Add the buttermilk and fold gently until a shaggy dough forms.
Turn dough onto a floured surface. Fold it over itself 3-4 times, then pat out to 3/4-inch thickness. Cut out biscuits using a 2.5-inch cutter (do not twist).
Arrange the cut biscuits in a ring around the outer edge of the skillet, resting directly on top of the warm gravy. Leave the center open.
Bake in the preheated oven for 15 to 20 minutes, until the biscuits are golden brown and the gravy is bubbling.
Let rest for 5 minutes before serving warm directly from the skillet.