Melt Salted Butter (3 1/2 tablespoon) in a saucepan over medium heat.
Add All-Purpose Flour (50 gram) and whisk rapidly to combine.
Reduce to low heat and continue to whisk as the mixture cooks into a paste (roux), about two minutes. Be careful not to let it burn.
Slowly begin incorporating Milk (500 milliliter), a splash at a time, whisking continuously.
Continue to add milk and cook until the mixture forms a paste, about ten minutes.
Once mixture is smooth and thick, take off heat.
Add in Mature Cheddar Cheese (75 gram), and Salt (to taste) and Ground Black Pepper (to taste).