Combine coconut milk, Hawaiian marinade, and garlic powder in a dish large enough to marinate the chicken breasts. Add chicken. Cover and marinate in the refrigerator, turning every 3 to 4 hours, about 24 hours.

Combine coconut milk, cinnamon, vanilla extract, and butter flavoring in a dish. Add pineapple. Cover and marinate in the refrigerator, at least 1 hour.

Preheat an outdoor grill for high heat and lightly oil the grate. Place chicken on hot grill; discard marinade. Cook chicken, without pressing or poking, and flipping only twice, until no longer pink in the center and the juices run clear, 5 to 7 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Transfer chicken to a platter; let rest for 4 to 5 minutes.

Place pineapple on the hot grill and cook on medium-high heat for 2 to 3 minutes per side. Transfer grilled pineapple to the platter with the chicken.