Heat the oven to 375°F. Lightly grease a 9×13-inch baking dish.
Cook the macaroni in a large pot of boiling water according to the package directions, stopping when it is al dente. Drain well, return it to the pot, and stir in the butter until melted. Let the pasta cool briefly while you prepare the milk mixture.
Whisk the evaporated milk, whole milk, eggs, and garlic salt in a bowl until smooth, with no streaks of egg white.
Combine the cheddar and mozzarella in another bowl. Spread one-third of the macaroni in the baking dish and cover with one-third of the cheese. Repeat twice, ending with cheese. Pour the milk mixture evenly over the layers and sprinkle with paprika.
Bake uncovered, starting to check after 25 minutes. Allow about 30 minutes, or longer as needed, until the top is golden, the center is set, and a food thermometer inserted in the middle reads 165°F. If the top browns too quickly, loosely cover it with foil while the center finishes cooking.
Let the macaroni and cheese rest for 10 minutes before scooping and serving.