01 - Preheat oven to 390°F (200°C). Lightly grease a medium baking dish with butter or oil.
02 - Bring a large pot of water to a boil and season with 1 tablespoon salt. Add elbow macaroni and cook until just al dente. Drain well and set aside.
03 - In a medium saucepan over moderate heat, melt 2 tablespoons unsalted butter. Add all-purpose flour and whisk, cooking for 1 minute until foamy.
04 - Gradually pour in whole milk, whisking constantly to eliminate lumps. Cook, stirring often, until the sauce thickens and coats the back of a spoon, about 4 to 5 minutes.
05 - Reduce the heat to low. Stir in cheddar, mozzarella, and cream cheese until melted and smooth.
06 - Blend in gochujang, Dijon mustard, smoked paprika, and black pepper. Add salt to taste.
07 - Gently fold in chopped kimchi, kimchi juice, and finely sliced spring onions until evenly distributed.
08 - Add the drained macaroni to the cheese mixture. Stir until each piece is thoroughly coated. Transfer mixture to the greased baking dish, spreading evenly.
09 - In a bowl, mix panko breadcrumbs, toasted sesame seeds, melted butter, and sesame oil. Sprinkle the mixture uniformly over the macaroni.
10 - Place dish in the oven and bake for 15 to 20 minutes, until the top is golden brown and the filling is bubbling. Let cool for a few minutes before portioning.