In a medium bowl, whisk together Dijon mustard, honey, olive oil, apple cider vinegar, minced garlic, salt, black pepper, paprika, and cayenne pepper (if using).
Place chicken breasts in a resealable plastic bag or shallow dish and pour the marinade over the chicken.
Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 24 hours for maximum flavor.
Preheat the grill to medium-high heat. Lightly oil the grill grates.
Remove chicken from the marinade and discard the excess marinade.
Grill chicken for about 6-7 minutes per side, or until the internal temperature reaches 165°F (75°C).
Remove from grill and let rest for 5 minutes before serving.