01 - Preheat oven to 400°F. Lightly grease a medium baking dish with butter or cooking spray.
02 - Cook elbow macaroni according to package directions until al dente. Drain thoroughly and set aside.
03 - In a saucepan over medium heat, melt 2 tablespoons butter. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth paste.
04 - Gradually pour in milk while whisking continuously. Simmer for 4-5 minutes until thickened. Reduce heat to low and stir in cheddar, mozzarella, Dijon mustard, black pepper, and salt until fully melted and smooth.
05 - Add cooked macaroni, chopped kimchi, kimchi juice, spring onions, and gochujang to the cheese sauce. Fold gently until evenly distributed.
06 - Pour the macaroni and cheese mixture into the prepared baking dish, spreading evenly.
07 - In a small bowl, combine panko breadcrumbs, melted butter, and sesame seeds. Mix until crumbs are evenly coated.
08 - Sprinkle breadcrumb mixture evenly over the macaroni. Bake for 15 minutes until golden brown and bubbling. For extra crispiness, broil for 2 minutes at the end.
09 - Remove from oven and let rest for 5 minutes to allow the sauce to set slightly before serving.