In a small saucepan, whisk together the milk, cornstarch, dry mustard, and salt.
Turn on the stove to medium-low and bring to a simmer. Let it gently bubble, stirring frequently, for 5-6 minutes or until thickened.
Reduce heat to low. Add the cheese a little bit at a time, stirring in completely after each addition.
Serve over baked potatoes, French fries, or as a dip for pretzels, chips, and more.
