Easy Cheese Sauce

Heat butter in a medium saucepan over medium heat until melted.

Add flour; cook, whisking constantly, until thickened and just starting to turn light brown, 2 to 3 minutes.

Slowly add milk, whisking constantly. Cook, stirring often, until the mixture is thick enough to coat the back of a spoon, 10 to 12 minutes. (Do not boil.)

Remove from heat and, a handful at a time, whisk in Cheddar and Parmesan until smooth. Stir in salt.

Transfer the sauce to a 4-cup heatproof container and let cool at room temperature, about 30 minutes. Refrigerate for up to 5 days.