Mix melted butter, chicken broth, vinegar, and garlic powder for the injection.
Inject the mixture evenly throughout the turkey breast.
Coat the entire surface with yellow mustard.
Combine paprika, brown sugar, salt, pepper, onion powder, and cayenne; apply the rub.
Preheat smoker to 250–275°F with wood or charcoal.
Smoke the turkey until internal temperature reaches 150°F.
Baste with melted butter, wrap tightly in foil, and continue cooking to 160–165°F.
Rest 15 minutes, then slice against the grain and serve.
Tip: Keep the smoker lid closed as much as possible to maintain steady temperature.