Place asparagus and hoisin sauce into a resealable plastic bag and shake several times to coat asparagus with sauce. Allow to stand at least 30 minutes. For best flavor, refrigerate and marinate overnight.
Preheat an outdoor grill for medium heat and lightly oil the grate.
Remove asparagus from bag and shake off excess hoisin sauce; lay asparagus spears onto the grill and cook, turning every 1 to 2 minutes, until all sides of the spears show grill marks and hoisin sauce has caramelized onto the asparagus, 4 to 6 minutes.
Transfer asparagus to a serving platter and sprinkle with sesame seeds to serve.
