Preheat the grill to 400 degrees.
Place the chicken on the grill and grill for 3-4 minutes. Flip, brush with the remaining reserved honey mustard sauce, and grill for 3-4 minutes more.
Flip and brush the other side with sauce. Grill until the sauce has caramelized and the internal temperature is 165 degrees, about 1-2 minutes.
Let rest, then dice into cubes.