Prepare. Gather the ingredients. Preheat the oven to 400° F and grease a large cast iron skillet (I use a 13.25" skillet). Or line a baking sheet with parchment paper.

Make the Buttermilk. Combine the mix and vinegar in a small bowl (or skip this step and use buttermilk). Set aside.

Prep the Butter. Grate the frozen butter on a box grater. Put the grated butter back into the freezer while you prep the rest of the ingredients.

Mix the Dry Ingredients. In a large mixing bowl, whisk together the flour, salt, sugar, baking powder, and baking soda.

Cut in the Butter. Cut in the frozen shredded butter with a biscuit cutter or fork until it resembles pea-sized crumbs. Don't overdo it here. The butter doesn't have to be fully combined.

Add the Wet Ingredients. Add the milk mixture and sourdough starter.

Combine into Dough. Mix with a spoon until combined. Then, use your hands to ensure no dry ingredients remain. The dough should come together to form a ball, but don't overwork it.

Cut the Biscuits. Turn out the biscuit dough onto a floured counter or work surface. Use your hands or a rolling pin to shape the dough 1" thick. Use a biscuit cutter or a glass to cut the biscuits. Reshape the leftover dough to cut more biscuits if needed.

Bake. Transfer the biscuits to the prepared skillet and bake for 25-30 minutes or until the tops are golden.

Brown the Sausage. While baking biscuits, brown the sausage in a large skillet over medium heat.

Add the Flour and Butter. Once the sausage is thoroughly cooked, add the butter and flour to the sausage. Stir to combine and cook for about a minute.

Add the Milk. Slowly pour in the milk. Bring to a gentle simmer while stirring constantly. Cook until the gravy thickens, about 5-10 minutes.

Season and Serve. Season with salt and pepper to taste. Serve over warm sourdough biscuits.