Bring a pot of salted water to a boil. Cook macaroni according to package instructions, draining it once it is just barely al dente. If you will be baking the final mac, take it out about 30-60 seconds earlier (since it will cook a bit more in the oven).

In a medium saucepan, combine milk and sodium citrate. Cook over medium heat, whisking occasionally, until sodium citrate is dissolved and milk just starts to form small bubbles around the edges (don't let it fully boil).

Reduce heat to low and add cheese, one handful at a time, whisking until each handful is fully melted before adding the next. If you will be finishing your mac and cheese in the oven, hold back about 1/4 cup of cheese for topping; otherwise you can mix all the cheese into the sauce.

Fold in drained macaroni, followed by kimchi.

Serve immediately, or, to finish it off in the oven, preheat broiler. Lightly butter 3 to 4 oven-safe cocottes or a 2 quart casserole dish. Pour macaroni into prepared dish. Sprinkle with reserved cheese and breadcrumbs.

Broil for 3 to 5 minutes or until breadcrumbs are browned and cheese is bubbly on the top (keep a close eye on it as the breadcrumbs will go from brown to burnt in the blink of an eye).

Let cool for 5 minutes before serving.