Combine the dark brown sugar, smoked paprika, sea salt, black pepper, garlic powder, onion powder, chili powder, dry mustard, cocoa powder, cumin, oregano, celery seed, allspice, and cayenne in a medium bowl.
Stir thoroughly, using the back of the spoon to break apart any packed clumps of brown sugar, until the color is even all the way through with no pockets of sugar or paprika.
Use right away, or transfer to an airtight jar and store in a cool, dark place for up to 6 months.
Notes
Yield: about 1 ¼ cups, enough to generously coat two racks of ribs (roughly ½ cup per rack).Heat level: ½ teaspoon of cayenne is mild. Use a full teaspoon for noticeable heat, or leave it out entirely.Binder: brush a thin coat of yellow mustard on the meat first so the rub sticks in an even layer. It will not taste like mustard.Storage: keep it in a cool, dark cabinet away from the stove. Measure into a small bowl rather than dipping used utensils into the jar.Beyond ribs: works on pork steaks, pork tenderloin, chicken, and burgers. For beef, cut the brown sugar in half and add more black pepper.