Combine soy sauce, half the green onion, garlic, sesame seeds, cornflour, ginger, mirin, sugar, chilli flakes and sesame oil in a bowl to make marinade.
Place prawns in a large resealable plastic bag; pour in marinade. Seal the bag and shake to coat prawns. Place in refrigerator and marinate for 15 to 20 minutes.
Heat a large frying pan over medium-high heat; add prawn. Cook, turning and basting a few times with leftover marinade, 5 to 7 minutes. Transfer prawns to a bowl, reserving oil in the pan.
Pour 3 tablespoons of the marinade and gochujang into the pan over medium heat. Cook and stir until thickened, about 2 minutes. Brush glaze over cooked prawns or use as a dip. Sprinkle the remaining green onion on top.