01 - Preheat oven to 400°F. Lightly grease a medium baking dish with butter or cooking spray.
02 - Cook macaroni according to package instructions until just al dente. Drain well and set aside.
03 - In a saucepan over medium heat, melt 2 tbsp butter. Whisk in flour and cook for 1 minute, stirring constantly to form a smooth paste.
04 - Gradually pour in milk while whisking constantly. Continue whisking until mixture is smooth and thickened, about 4–5 minutes.
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 - Fold in the cooked macaroni, chopped kimchi, kimchi juice, spring onions, and gochujang. Mix until evenly combined.
07 - Pour the mac and cheese mixture into the prepared baking dish, spreading evenly.
08 - In a small bowl, combine panko breadcrumbs with 1 tbsp melted butter and sesame seeds. Mix until breadcrumbs are evenly coated.
09 - Sprinkle the breadcrumb mixture evenly over the macaroni.
10 - Bake for 15 minutes, or until topping is golden brown and cheese is bubbling around the edges.
11 - Let rest for 5 minutes before serving to allow sauce to set slightly.