Mix salt, black pepper, white pepper, cayenne, and garlic powder together in a small bowl. Set spice rub aside.

Stir vinegar and Dijon mustard together in a small bowl.

Place ribs on a foil-lined baking sheet. Turn the ribs over so that the meat side is down. Use the tip of a small, sharp knife to make very shallow slashes every few inches through the membrane that covers the ribs. Poke the knife 3 or 4 times between each rib bone, about 1/4 inch deep.

Brush about half the Dijon mustard mixture on top. Sprinkle with about 40% of the spice rub. Turn the rack over and brush on the rest of the mustard and vinegar mixture. Sprinkle on the rest of the rub, reserving 1 or 2 teaspoons for potential seasoning later.

Refrigerate the ribs, uncovered, for 4 to 12 hours before baking.

Preheat the oven to 300°F (150°C). Remove ribs from the refrigerator.

Cook ribs in the center of the preheated oven for 1.5 hours. Remove from the oven and baste with the pan drippings. Continue to cook until very tender, for about another 1.5 hours.