Preheat the oven to 375°F. Generously butter a 9-x-13-inch glass baking dish; set aside (I like to use my tagine since it goes beautifully from oven to table). Bring a large pot of salted water to a boil. Add the macaroni and according to package directions until al dente. Drain well and return the pasta to pot. Add the 4 tablespoons butter and toss until pasta is coated and butter has melted; set aside.
In a medium bowl, whisk together the evaporated milk, whole milk, and eggs. Stir in the seasoning salt, garlic powder, 1 teaspoon salt, and ½ teaspoon pepper; set aside. In another medium bowl, combine cheddar and Jack cheeses; set aside.
Spread one third of the macaroni in an even layer in the bottom of the buttered baking dish or tagine; cover evenly with one third of the cheese. Repeat with the remaining macaroni and shredded cheeses. Pour the milk mixture evenly over the contents of the baking dish, sprinkle with paprika, and bake until the top layer is lightly browned, 40 to 45 minutes. Let stand 10 to 15 minutes before serving.