Prepare the marinade by combining water, soy sauce, sesame seeds, molasses, vinegar, green onions, garlic powder, and cayenne pepper in a separate bowl.

Pour half of the marinade over the roast, ensuring it is well coated. Cover and refrigerate overnight.

Transfer the marinated roast and remaining marinade to a slow cooker. Cook on high for 1 hour, then reduce to low and cook for an additional 8-9 hours until the meat is tender.

Remove the meat and keep it warm. Skim any fat from the cooking juices, then thicken the sauce by stirring in cornstarch mixed with cold water. Pour the sauce over the meat and sprinkle with additional sesame seeds if desired.