Melt the butter in a large saucepan over medium heat. Whisk in the flour and continue whisking for approximately 1 minute until a thick paste is formed.
Then gradually whisk in the milk and cook uncovered until the mixture has thickened.
Remove the pan from the heat and then whisk in the cheddar cheese and salt. Whisk until the cheese is melted and the mixture is smooth.
Serve immediately and store leftovers in an air tight container in the refrigerator for up to one week.
Notes
If the cheese sauce is too thick, add in more milk gradually until the cheese sauce is the consistency that you prefer.