In a large zip-top bag, combine 1 cup buttermilk, 1/3 cup Buffalo sauce, 1 tablespoon honey, 1 teaspoon garlic powder, and 1 teaspoon black pepper; seal and mix until uniform.
Add the 6 boneless skinless chicken thighs to the bag, press out excess air, and refrigerate to marinate for 2 to 8 hours.
About 15 minutes before grilling, preheat the grill to medium-high heat.
Make the glaze by stirring 1/4 cup Buffalo sauce with 2 tablespoons honey in a small bowl until smooth.
Remove chicken from the marinade and discard excess marinade. Place thighs on the preheated grill and cook 5 to 7 minutes on the first side.
Flip the thighs, brush the grilled side with the buffalo-honey glaze, and cook 5 to 7 more minutes.
Flip once more, brush again with glaze, and continue cooking until an instant-read thermometer inserted into the thickest part reads 170°F (about a few more minutes as needed).
Remove from the grill and let rest briefly before serving.