Stir the onion powder, garlic powder, rosemary, paprika, cayenne, fennel, cocoa, sea salt, and black pepper together in a small bowl. The mixture should look evenly blended, with no pockets of cocoa or concentrated spice.

Press the rub onto every side of the shoulder, then place the seasoned roast in a 5- or 6-quart slow cooker. A consistent coating gives the exposed surfaces an even layer of savory seasoning.

Pour the chicken broth into the cooker, cover, and select high for 4 hours or low for 6 hours. At the end of cooking, the pork should yield easily to gentle fork pressure.

Separate the tender pork into strands with two forks inside the crock, then switch the cooker to warm and leave the meat in its juices for 20 to 30 minutes. The shreds should become glossy and evenly coated with the seasoned liquid.

Spoon the pulled pork into the lettuce leaves for serving. Keep each filling centered so the crisp edges remain easy to fold around the meat.