Bring a large pot of water to a boil; add salt. Add pasta; cook until al dente. Drain well. Transfer to a large bowl.
Heat milk in a medium saucepan over medium heat until hot. Melt butter in a deep skillet over medium heat.When butter begins to foam, add flour; cook, whisking, 1 minute. Gradually whisk in hot milk. Cook, whisking constantly, until mixture starts to bubble and thicken, about 5 minutes.
Remove skillet from heat. Add 1 teaspoon salt, a pinch of black pepper, cayenne,mustard, and cheese; stir until melted.
Stir cheese sauce into the pasta, and serve immediately.