Preheat oven to 425°F. Grease a 13- x 9-inch baking dish with butter, and set aside.
Prepare macaroni according to package directions for al dente. Drain and transfer to a large bowl.
Stir butter into hot macaroni until melted. Stir in evaporated milk, whole milk, and eggs until well combined. Fold in pro cessed cheese, salt, pepper, 2 ½ cups of the sharp Cheddar, and 1 cup of the mild Cheddar.
Transfer macaroni mixture to prepared baking dish. Sprinkle evenly with remaining ½ cup each sharp Cheddar and mild Cheddar.
Bake, uncovered, in preheated oven until golden and bubbly, 45 to 50 minutes. Serve immediately.
