Cook pasta al dente. Strain. Rinse pasta with cold water so it’s less likely to stick together. Set pasta aside.
Preheat oven to 350 F.
Set a large pot to low heat.
Add the milk and salt. Gradually add both cheeses, stirring constantly, preferably with a wire whisk, until sauce is smooth.
Add pasta and stir together until well mixed. Remove from heat.
Gently fold in any extras (chilies, shredded pork, bacon, hot dogs, etc).
In a separate bowl, mix the melted butter into the panko until all the panko is moistened.
Put mac & cheese into a baking dish (or several small baking dishes) and cover the top with the buttered panko.
Bake at 350 F until the top is crispy, about 20 minutes.