Preheat the Oven: Set your oven to 350°F and grease a 9×13 baking dish.
Cook the Macaroni: Boil the macaroni in salted water until al dente, about 1-2 minutes less than the package instructions suggest. Drain well.
Combine Ingredients: In the prepared baking dish, combine the cooked macaroni with butter, onion powder, salt, pepper, milk, mozzarella, and 3 cups of cheddar. Mix well.
Add Eggs: In a separate bowl, beat the eggs and then stir them into the macaroni mixture.
Bake: Cover the dish with greased aluminum foil and bake for 45 minutes.
Final Touch: Remove the foil, stir the mixture, and sprinkle the remaining cup of cheddar on top. Bake uncovered for an additional 15 minutes until bubbly and golden.