In a medium pot, bring 6 cups of water to a rolling boil. Add 1 tsp salt and the 2 cups of elbow macaroni. Cook according to package directions for al dente (about 7–9 minutes), stirring occasionally so pasta doesn't stick. Reserve 1/4 cup of the starchy pasta cooking water, then drain the pasta and set aside.

Make the roux

In the same pot (or a clean medium saucepan) melt 2 tbsp unsalted butter over medium heat. Once melted and slightly bubbling, add 2 tbsp all-purpose flour and whisk continuously for 1–1.5 minutes until the mixture becomes a pale paste and loses its raw flour smell. Do not let it brown.

Add milk

Slowly pour in 1¾ cups whole milk while whisking constantly to avoid lumps. Continue to cook and whisk over medium heat until the sauce thickens and coats the back of a spoon, about 3–5 minutes. If it becomes too thick, whisk in up to the reserved 1/4 cup pasta water a tablespoon at a time to reach a smooth, pourable consistency.

Finish cheese sauce

Reduce heat to low. Whisk in 2 tbsp softened cream cheese until smooth. Gradually add 2 cups grated sharp cheddar cheese, a handful at a time, stirring each addition until melted and smooth before adding more. Stir in 1/2 tsp salt, 1/4 tsp black pepper (optional), and 1/4 tsp mustard powder (optional) and taste; adjust seasoning gently since cheddar can be salty.

Combine pasta and sauce

Add the drained pasta to the cheese sauce and gently fold until all pasta is evenly coated. If the sauce seems tight, add another tablespoon of reserved pasta water or milk to loosen. Keep warm over very low heat for 1–2 minutes while preparing topping (if using).

Topping (Optional, kid-friendly)

Prepare topping

Preheat the oven broiler to low or preheat a 350°F oven if you prefer a gentle bake. In a small bowl, mix 1/2 cup plain breadcrumbs (or crushed crackers) with 1 tbsp melted butter and 2 tbsp grated parmesan (if using) until evenly moistened.

Add topping and finish

If you want a warm, crispy top, transfer the mac & cheese to an oven-safe dish, sprinkle the breadcrumb mixture evenly over the surface, and place under the broiler for 1–2 minutes until golden (watch closely to avoid burning). Alternatively bake at 350°F for 8–10 minutes until bubbly and lightly browned. For a stovetop quick finish, sprinkle topping on and serve without broiling.