Meanwhile, melt butter in 3-quart saucepan over low heat. Stir in flour, salt, pepper, mustard and cayenne powder. Cook over medium low heat, stirring constantly, until mixture is smooth and bubbly. Then, remove the pan from heat and stir in the milk. Return the pan to the burner and heat to boiling, stirring constantly. Once boiling, stir for one minute and then remove from heat. Stir in the cheese, and cook until it's melted and smooth.