In a mixing bowl, combine the softened butter, buttermilk, garlic powder, onion powder, smoked paprika, cayenne pepper (if using), salt, and black pepper. Stir until you achieve a smooth, creamy mixture. This will be your marinade.
Place the chicken breasts in a resealable plastic bag or a shallow dish, and pour the butter mixture over the chicken. Make sure each piece is well-coated. Seal the bag or cover the dish, and refrigerate for at least 2 hours, or overnight for best flavor.
Preheat your grill or oven to medium-high heat (around 375°F/190°C).
Remove the chicken from the marinade, allowing any excess to drip off. Reserve the leftover marinade for basting.
Grill the chicken for about 6-7 minutes on each side, or until fully cooked (internal temperature should reach 165°F/75°C). Baste with the reserved marinade occasionally for extra moisture and flavor.
Once cooked, remove the chicken from the grill and let it rest for 5 minutes before slicing.
Garnish with chopped fresh parsley and serve with lemon wedges on the side for a zesty touch.