Preheat oven to 400 degrees F. On a baking sheet lined with foil or parchment paper, bake bacon until crispy, 12-15 minutes. Remove bacon and let it drain a few paper towels. Roughly chop.
Cook pasta according to package. Drain and set aside.
In a large pot, melt 4 tablespoons of the unsalted butter and whisk in flour over medium-high heat. Cook for a few minutes until the mixture turns a light tan color, then gradually add milk and whisk until sauce thickens, maybe five minutes.
Add cheddar cheese and whisk until smooth. Then add salt, pepper, bacon, kimchi, and cooked macaroni. Stir together well.
Melt remaining butter and stir together with breadcrumbs and cayenne pepper.
Transfer macaroni mixture to a 13×9-inch baking dish and top with buttered panko. Bake mixture until golden brown, 10-15 minutes. Serve while hot.