Stir the sugar into the soy sauce in a bowl until the sugar dissolves completely. Whisk the sesame oil, garlic, green onions, and sesame seeds into the soy sauce. Place the short ribs in a large, sealable bag and pour the marinade over the meat. Marinate in refrigerator for 8 hours or overnight.
Preheat an outdoor grill for medium heat, and lightly oil the grate. Remove the ribs from the bag; discard the marinade.
Cook the short ribs on the preheated grill until they are very firm, hot, and grey in the center, about 15 minutes per side. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).