In a fondue pot (or you can do this in a saucepan on the stove), melt the butter on low.
Add in your cornstarch, stirring well.
Add the salt.
Add in your milk, and stir constantly over medium-low heat until it begins boiling. Continue boiling and stirring for about a minute. Whatever you do, don't stop stirring! If you do, it will stick to the bottom of the pan and you will have a mess.
Turn the heat down to low and add your cheese, 1/2 cup at a time. Stir it well and fully melt after each addition.
Serve warm with your dippers! I made ours in the fondue pot and set it to warm and it stayed perfect for a few hours!