01 - Preheat oven to 400°F (200°C) and lightly grease a medium baking dish.
02 - Bring a large pot of water to a rolling boil, season generously with salt, and cook elbow macaroni until just al dente. Drain pasta and set aside.
03 - In a medium saucepan over medium heat, melt unsalted butter. Whisk in flour and cook, stirring constantly, for 1 minute until fragrant.
04 - Gradually add whole milk to the roux, whisking continuously until mixture is smooth and thickens, approximately 4–5 minutes.
05 - Reduce the heat and add grated cheddar, mozzarella, and cubed cream cheese, stirring until fully melted and incorporated.
06 - Mix in gochujang, Dijon mustard, smoked paprika, and black pepper, then adjust salt to taste.
07 - Fold in chopped kimchi, kimchi juice, and sliced spring onions until evenly distributed throughout the sauce.
08 - Add drained macaroni to the cheese and kimchi mixture, stirring thoroughly until all pasta is well coated. Transfer mixture to the prepared baking dish.
09 - In a mixing bowl, combine panko breadcrumbs, toasted sesame seeds, melted butter, and sesame oil. Distribute topping evenly over the pasta layer.
10 - Bake in the preheated oven for 15 to 20 minutes, until the topping is golden and the dish is bubbling around the edges. Allow to cool briefly before serving.