Preheat oven to 350ºF (177ºC). Spray a 9x13-inch baking dish with nonstick cooking spray; set aside.
Cook pasta (2 cups) in well-salted water, according to the package instructions, just until al dente.
Meanwhile, in a medium mixing bowl stir together all the rest of the ingredients, except the cheeses.
Drain the pasta. Return drained pasta immediately to the pot it was just cooked in. Fold in the cheddar (8 ounces), American cheese (8 ounces), and cream cheese (4 ounces) until evenly combined.
Pour the pasta and cheese mixture into the prepared 9x13-inch baking dish. Pour the whisked ingredients evenly over the pasta.
Bake in the preheated oven until bubbly, 40-45 minutes. Let stand for 10 minutes prior to serving.
Notes
The secret is the custard: Unlike stovetop mac and cheese, this one bakes into a rich, creamy casserole that holds its shape while staying luscious inside.
Cheese mix matters: Don’t skip the American cheese — it melts smoother than cheddar alone and creates that signature creaminess.
Avoid grainy texture: Add the custard mixture evenly over the hot pasta instead of dumping it all at once.
Add-ins? If you like a little extra flavor, stir in a handful of cooked bacon bits, sautéed onions, or even roasted jalapeños.
Make-ahead magic: Assemble up to a day ahead, refrigerate, and bake just before serving. Perfect for holidays!