In a pan cook butter, salt, pepper, and flour and stir until smooth.
Remove from heat and add milk, return to heat and bring to a boil.
Boil 1 minute. You should notice it start to thicken.
Remove from heat and add cheese, stir until melted. This is where I add the mustard and shredded cheddar to taste. Start small and add until you get the extra flavor that you like.