Bring a large pot of salted water to a boil. Add the elbow macaroni and cook according to package instructions until al dente, usually about 7-8 minutes. Drain the pasta and set aside.
In the same pot or a separate saucepan, melt 1/4 cup of unsalted butter over medium heat. Once melted, whisk in the all-purpose flour and cook for about 1-2 minutes until you have a smooth, bubbly roux. Gradually add the 3 cups of milk, whisking constantly to avoid lumps. Cook the mixture until it thickens, about 5-7 minutes.
Lower the heat and stir in garlic powder, onion powder, salt, and black pepper to taste. Slowly add the shredded cheddar and mozzarella cheeses, stirring continuously until melted and smooth.
Return the drained elbow macaroni to the pot with the cheese sauce. Stir well to coat the pasta evenly.
In a mixing bowl, combine the crushed Ritz crackers with 2 tablespoons of melted butter. Mix until the cracker crumbs are evenly coated.
Pour the cheesy macaroni mixture into a greased 9x13-inch baking dish. Evenly sprinkle the buttery Ritz cracker topping over the surface. Bake in a preheated oven at 350°F (175°C) for 25-30 minutes, or until the topping is golden brown and crispy.
Let the baked mac and cheese cool for a few minutes before serving. This allows the sauce to thicken slightly and makes it easier to portion out.
