In a large saucepan over medium heat, combine elbow macaroni and enough whole milk to cover the pasta. Stir frequently for about 8-10 minutes, ensuring the mixture comes to a gentle simmer. Keep an eye on it; the pasta should be tender and most of the milk absorbed by the time you’re done.

Once the pasta is cooked, reduce the heat to low and add in the unsalted butter. Stir until melted, then gradually mix in the shredded sharp cheddar, mozzarella, and grated Parmesan cheese. Continue stirring until everything is smoothly combined and melted, which should take approximately 3-4 minutes.

If you’re including sour cream for an extra layer of richness, gently fold it into the cheesy pasta mixture. Then, season with salt and pepper to taste, ensuring a perfect balance of flavors. Stir well for about 2 minutes until everything is evenly coated.

If you desire a crunchy topping, sprinkle breadcrumbs evenly across the mac and cheese. Preheat your oven's broiler, then broil on high for 2-3 minutes, or until the breadcrumbs turn golden brown and crispy.

Carefully remove the mac and cheese from the heat, and serve it hot in bowls or on plates. Garnish with freshly chopped parsley for a pop of color.