Prepare a grill fire to 325° using hickory or oak for smoke flavor.
Drizzle olive oil over chicken thighs and season with bbq rub.
Roll the chicken thighs up and place on a rimmed sheet pan.
Drizzle with olive oil and season with additional rub.
Make the Cowboy Butter: Melt butter in a saucepan and add minced garlic, cooking 2- 3 minutes. Remove from heat and add horseradish, mustard, thyme, salt, pepper and chili flakes. Stir to combine. Set aside.
Grill the chicken thighs over indirect fire with the grill lid closed.
Cook for 20 minutes.
Brush with cowboy butter and continue cooking for 5 – 10 minutes or until chicken reaches 165° using a digital meat thermometer.
Remove from grill.
Serve with additional cowboy butter if desired.