Preheat the oven to 400 degrees F (200 degrees C).
Mix garlic powder, paprika, mustard, parsley, salt, and pepper in a small bowl. Rub all sides of each chicken breast in olive oil and coat all sides with the spice rub. Place on a baking sheet. Top each breast with 1 tablespoon of butter. Cover each breast with parchment paper and tuck the paper underneath, creating a sealed pouch.
Bake until chicken is no longer pink in the center and the juices run clear, about 35 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Remove chicken from the oven and let rest 5 minutes.