Cook the 1 lb elbow macaroni in salted boiling water for 1 minute less than the package directions. Drain and set aside without rinsing.

Melt the 3 tbsp butter over medium heat in a large saucepan. Add the 3 tbsp flour and stir for 90 seconds until pale golden.

Pour in the 2 cups whole milk and 1 cup heavy cream in a slow stream, whisking constantly. Cook over medium heat for 5 to 7 minutes until the sauce coats the back of a spoon.

Reduce to low heat. Stir in the 1 cup American cheese until melted, then add the 1½ cups sharp cheddar and ½ cup parmesan. Stir for 2 minutes until smooth. Add the 1 tsp salt, ½ tsp garlic powder, ½ tsp mustard powder, and ¼ tsp white pepper.

Fold in the drained macaroni until fully coated. Transfer to a lightly greased 9x13-inch baking dish.

Bake uncovered at 375°F (190°C) for 20 to 25 minutes until the top is golden and the edges bubble. Rest for 3 to 4 minutes, then scoop and serve.