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