01 - Preheat oven to 390°F. Grease a medium baking dish evenly with butter.
02 - Boil elbow macaroni in salted water until just al dente. Drain thoroughly and set aside.
03 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook, stirring, for 1 minute until mixture bubbles but does not brown.
04 - Gradually add milk, whisking constantly until sauce is smooth and thickened, about 4–5 minutes.
05 - Lower heat and stir in cheddar, mozzarella, and cream cheese. Mix until fully melted and silky.
06 - Blend in gochujang, Dijon mustard, smoked paprika, and black pepper. Season with salt as preferred.
07 - Fold in chopped kimchi, kimchi juice, and spring onions to the cheese sauce.
08 - Add drained macaroni to the sauce and fold gently to coat evenly. Transfer mixture to the prepared baking dish.
09 - In a bowl, combine panko breadcrumbs, sesame seeds, melted butter, and sesame oil. Stir well, then sprinkle evenly over the macaroni.
10 - Bake for 15–20 minutes until topping is golden brown and edges are bubbling. Allow to cool slightly before serving.