01 - Preheat oven to 390°F and lightly grease a medium-sized baking dish.
02 - Boil elbow macaroni in salted water until just al dente, then drain and set aside.
03 - In a medium saucepan over medium heat, melt unsalted butter. Whisk in flour and cook, stirring, for 1 minute until aromatic.
04 - Gradually pour in whole milk, whisking continuously until mixture thickens and becomes smooth, about 4 to 5 minutes.
05 - Reduce heat to low, then stir in cheddar, mozzarella, and cream cheese until fully melted and blended.
06 - Incorporate gochujang, Dijon mustard, smoked paprika, ground black pepper, and a pinch of salt. Mix thoroughly.
07 - Fold in the chopped kimchi, kimchi juice, and sliced spring onions for layered flavor.
08 - Add the cooked macaroni to the cheese sauce, stirring until pasta is completely coated. Transfer to the greased baking dish.
09 - Combine panko breadcrumbs, toasted sesame seeds, melted butter, and sesame oil in a bowl. Evenly sprinkle over the pasta mixture.
10 - Bake uncovered for 15–20 minutes until top is golden and filling is bubbling. Allow to cool briefly before serving.