01 - Preheat oven to 400°F. Lightly grease a medium baking dish with butter or cooking spray.
02 - Cook macaroni according to package directions until al dente. Drain thoroughly and set aside.
03 - Melt 2 tbsp butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth paste.
04 - Gradually pour in milk while whisking continuously. Cook for 4-5 minutes until sauce thickens and coats the back of a spoon.
05 - Reduce heat to low. Stir in cheddar, mozzarella, Dijon mustard, black pepper, and salt until cheese is completely melted and sauce is smooth.
06 - Add cooked macaroni, chopped kimchi, kimchi juice, spring onions, and gochujang (if using). Fold until pasta is evenly coated.
07 - Pour mixture into the prepared baking dish, spreading evenly.
08 - Combine panko breadcrumbs, melted butter, and sesame seeds in a small bowl.
09 - Sprinkle breadcrumb mixture evenly over the mac and cheese.
10 - Bake for 15 minutes until golden brown and bubbly. For extra crispiness, broil for the last 2 minutes.
11 - Let rest for 5 minutes before serving to allow the sauce to set.