Bring a large pot of salted water to a rolling boil, then add 1 pound of elbow macaroni and cook for 8-10 minutes until al dente. Drain and set aside.

Melt 4 tablespoons of unsalted butter in a medium saucepan over medium heat. Whisk in ¼ cup of all-purpose flour to make a roux. Gradually add 4 cups of whole milk, whisking until thickened (about 5-7 minutes).

Remove from heat and stir in 2 cups of sharp cheddar, 1 cup of Monterey Jack, and ½ cup of Parmesan cheese until melted and smooth.

Fold the drained macaroni into the cheese sauce, ensuring all is well coated. Add in mix-ins if desired.

Preheat oven to 350°F (175°C). Grease a 3-quart baking dish, pour in the macaroni and cheese mixture, spreading evenly. Mix 1 cup of panko breadcrumbs with 2 tablespoons of melted butter and sprinkle on top.

Bake for 25-30 minutes until the top is golden brown and cheese is bubbly. Let rest for 5 minutes before serving.