Cook pasta in heavily salted boiling water until 2 minutes shy of package directions. Drain and set aside.

Undercooking is essential—pasta finishes cooking in the oven • Pasta has slight white center when bitten • Pasta is tender but firm

While pasta cooks, warm milk in microwave or small saucepan until hot but not boiling.

Warm milk prevents lumps when added to roux • Steam rising from milk

Melt butter in a large pot over medium heat. Whisk in flour. Cook, whisking constantly, for 2 minutes until mixture smells like biscuits and is foamy.

Do not let it brown—we want a white roux for light-colored sauce • Mixture is smooth, foamy, light golden color • Nutty, toasted flour aroma

Slowly pour warm milk into roux, whisking constantly to prevent lumps. Add mustard powder, cayenne, and salt (1 tsp). Simmer, whisking occasionally, until sauce thickens enough to coat the back of a spoon (about 5 minutes).

Add milk gradually—rushing creates lumps • When you drag a finger across the back of a sauce-coated spoon, the line holds • Sauce bubbles gently, smells creamy and nutty

Remove pot from heat completely. Whisk in cheese one handful at a time, letting each addition melt before adding more.

Off the heat is critical—cheese separates above 180°F • Sauce is creamy yellow-orange, no visible cheese shreds • Sauce is velvety smooth, no grittiness

Fold cooked pasta into the cheese sauce until evenly coated. Pour into a buttered 9x13-inch baking dish.

Fold gently to avoid breaking pasta • Pasta is swimming in creamy sauce

Sprinkle remaining cheese on top. Bake at 375°F (190°C) for 20-25 minutes until bubbly and golden on top.

Let rest 5 minutes after baking—sauce thickens as it cools • Cheese on top is melted and lightly browned, sauce bubbles at edges • Bubbling sounds, cheese aroma