Bring a large pot of well-salted water to a rolling boil. Add elbow macaroni and cook until just al dente. Drain thoroughly and set aside. Do not rinse the pasta.

In a large saucepan over medium heat, melt butter until shimmering. Add flour and whisk constantly for 1 to 2 minutes until the mixture turns pale golden and smells slightly nutty.

Slowly pour in whole milk about half a cup at a time, whisking constantly after each addition. Cook over medium heat, stirring frequently, for 5 to 7 minutes until the sauce thickens and coats the back of a spoon.

Remove saucepan completely from heat. Add grated sharp cheddar and optional mild cheddar or American cheese. Stir in salt, pepper, dry mustard powder, and nutmeg if using. Stir until fully melted and sauce is smooth and glossy.

Add drained macaroni to the cheese sauce and stir gently until every piece is evenly coated. Taste and adjust salt if needed. Serve immediately.