How to Make It
Preheat oven to 250°F. Fit a wire rack inside a rimmed baking sheet.
Pat rib slabs dry using a paper towel. Trim and remove membrane from underside of ribs. Rub ribs all over with 2 tablespoons of the mustard. Stir together salt and black pepper in a small bowl; rub evenly over both sides of ribs.
Wrap each rib slab in aluminum foil; seal tightly. Place on wire rack set in baking sheet. Bake in preheated oven 2 1/2 hours. Remove from oven; remove and discard foil, and place ribs to face meaty sides up on wire rack. Return to oven, and bake at 250°F until a skewer or wooden pick can be pushed into meat with minimal resistance, about 1 1/2 hours.
Meanwhile, stir together ketchup, brown sugar, bourbon, vinegar, soy sauce, Worcestershire, cayenne, and remaining 1 teaspoon mustard in a medium saucepan. Bring to a boil over high, stirring occasionally. Reduce heat to medium-low; simmer, stirring occasionally, until thickened, 15 to 20 minutes.
Increase oven temperature to 500°F. Brush each rib slab with about 1/2 cup ketchup mixture. Return to oven, meaty sides facing up; bake at 500°F until meat is well browned and sauce begins to caramelize, 5 to 10 minutes. Let stand 5 minutes. Serve.