Preheat the oven to 400 degrees F. Butter a shallow 2-quart baking dish; set aside. Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs for al dente. Reserve 1 cup cooking water, then drain.
Meanwhile, melt 4 tablespoons butter in a large saucepan over medium heat. Whisk in the flour and cook, whisking, until smooth and lightly browned, about 2 minutes. Gradually whisk in the milk until smooth. Add the bay leaf, cayenne and 1 teaspoon salt. Cook, whisking occasionally, until the sauce comes to a simmer and thickens slightly, 7 to 8 minutes.
Remove the bay leaf from the sauce and whisk in the monterey jack, gruyere and cheddar one cheese at a time, allowing each cheese to melt before adding more. Add the pasta and stir until coated, adding the reserved cooking water as needed to loosen (the sauce will thicken as it bakes).
Transfer the pasta mixture to the prepared baking dish. Melt the remaining 2 tablespoons butter in the microwave. Add the panko, parmesan and a pinch of salt; stir to combine. Scatter over the pasta and bake until the crumbs are browned and crisp, 15 to 20 minutes.
The eggs in this mac and cheese make it fully set like a quiche.