Bring a large pot of salted water to a rolling boil and add the macaroni. Cook it about one minute less than the package suggests, since it will keep cooking in the oven later. Drain it well in a colander, then toss it with a little butter so it doesn't clump together while you prepare the sauce.
In your saucepan, melt the butter over medium heat until it's foamy but not browned. Whisk in the flour and let it cook for about a minute, stirring constantly, until it smells slightly toasty and turns a pale golden color. This step is what gives the final sauce its body.
Slowly pour in the warmed milk while whisking, then add the cream. Keep whisking over medium heat until the mixture thickens enough to coat the back of a spoon, roughly five to six minutes. Lower the heat, then stir in the cheddar, gruyere, and mozzarella a handful at a time until it's fully melted and glossy.
Pour the cheese sauce over the drained macaroni and fold everything together gently with a spatula. Season with salt, pepper, and smoked paprika, tasting as you go. Make sure every piece of pasta gets coated. This is where the magic of a creamy baked mac and cheese recipe really comes together.
Transfer the mixture into your buttered baking dish and spread it out evenly. Mix the panko with the melted butter until it looks like wet sand, then scatter it evenly across the top. This layer is what gives you that irresistible crunchy contrast against the creamy inside.
Bake uncovered at 375°F for about 25 to 30 minutes, until the top is deeply golden and the edges are bubbling. Let it rest for five minutes before serving so the sauce has time to settle and thicken slightly.