Preheat your oven to 400°F (200°C).
Bring a large pot of salted water to a boil.
In a small saucepan, gently heat the milk and bay leaves over medium-low heat until small bubbles form around the edges, about 5 minutes.
Remove from heat and let the bay leaves infuse the milk.
Cook the pasta in the boiling water until it's just shy of al dente—it should still have a noticeable bite.
Drain the pasta, give it a quick rinse with cool water to stop the cooking, and set it aside in a large bowl.
In a medium saucepan, melt 3 tablespoons (45 ml / 1.5 fl oz) of the butter over medium-low heat.
Whisk in the flour and cook, stirring constantly, for about 3-5 minutes to create a light brown roux.
Remove the bay leaves from the milk. Gradually whisk the warm milk into the roux, a little at a time, ensuring the sauce is smooth before adding more.
Continue until all the milk is incorporated and the sauce is thick and creamy.
Remove the sauce from the heat and stir in the grated cheddar until completely melted and smooth.
Pour the cheese sauce over the cooked pasta.
Add the Parmesan cheese, a generous pinch of salt, and a few grinds of black pepper.
Stir everything together until the pasta is evenly coated.
Grease a 9x13-inch (or similar) baking dish with the remaining 1 tablespoon (15 ml / 0.5 fl oz) of butter.
Pour the pasta mixture into the dish and spread it evenly.
Sprinkle the bread crumbs liberally over the top.
Bake for 15-20 minutes, or until the sauce is bubbling and the breadcrumb topping is golden brown and crispy.
Let it rest for a few minutes before serving hot.
Grate your own cheese. Pre-shredded cheeses contain anti-caking agents that can make your sauce grainy. A block of cheese will melt much more smoothly.
Don't skip the roux.
Cooking the flour and butter for a few minutes removes the raw flour taste and is the key to a thick, stable cheese sauce.
