Using a fork, combine the butter ingredients. Divide the seasoned butter into 8 equal portions.
Prepare the grill for indirect and direct cooking over medium-high heat.
Pat chicken thighs dry with paper towels. Using your fingertips, gently loosen the skin of the thighs, being careful not to completely separate the skin from the meat. Lift skin up and place a portion of butter underneath. Smooth the skin over the butter and massage it gently to spread it evenly over the top of the meat. Season thighs on both sides with salt and pepper.
Grill thighs, skin side up first, over indirect medium-high heat, with the lid closed, until sizzling, slightly firm and lightly marked on the bottom, 18 to 20 minutes. Then turn the thighs over and cook over direct medium-high heat, with the lid closed, until the juices run clear, the skin is blistered and crisp, and the meat is no longer pink at the bone, 2 to 4 minutes more. If flare-ups occur (and they did frequently for me), move the thighs temporarily over indirect heat. Remove from grill and let rest for 3 to 5 minutes. Serve warm.