Step 1: Prepare Cheese Topping. In a small bowl, combine ½ cup each of the shredded sharp cheddar, Colby Jack, smoked Gouda, and smoked Gruyère. Set aside for the topping.
Step 2: Preheat Oven & Prepare Dish. Preheat your oven to 350°F (177°C). Lightly grease a 9x13x3-inch casserole dish.
Step 3: Cook Pasta. Bring 4 liters of water to a rolling boil in a large pot. Add 1½ tablespoons of kosher salt. Cook the large elbow macaroni according to package directions, or for about 10 minutes, until al dente. Drain thoroughly and set aside.
Step 4: Prepare Sauce Base. In a medium bowl, whisk together the gently beaten egg, evaporated milk (or whole milk), seasoned salt, granulated garlic, granulated onion, ground mustard, ground paprika, and optional cayenne pepper until smooth.
Step 5: Combine Pasta & Liquid. Return the drained macaroni to the same large pot. Over medium heat, melt the unsalted butter in the pot. Stir the macaroni into the melted butter until coated. Pour the milk and spice mixture over the pasta and stir well to combine.
Step 6: Add Remaining Cheeses. Add the shredded Velveeta, and the remaining shredded sharp cheddar, smoked Gouda, smoked Gruyère, and Colby Jack cheeses to the pot. Stir until all cheeses are fully melted and combined into a luscious, gooey sauce.
Step 7: Assemble Casserole. Transfer the cheesy macaroni mixture into your prepared casserole dish. Spread it out evenly. Sprinkle the reserved cheese mixture from Step 1 generously over the top.
Step 8: Bake Covered. Cover the casserole dish with aluminum foil (lightly spray the inside of the foil with non-stick cooking spray to prevent sticking). Bake for 30 minutes.
Step 9: Bake Uncovered. After 30 minutes, carefully remove the foil. Bake uncovered for an additional 25-30 minutes, or until the top is bubbly, golden brown, and stunning.
Step 10: Rest & Serve. Let your Five Cheese Macaroni sit for 10 minutes before serving. This allows the sauce to set slightly, making it even creamier and easier to serve. Enjoy!