Melt butter in a heavy saucepan over low heat; add flour, stirring with a wire whisk until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and pepper.
Note: For a thinner white sauce, delete 1 tablespoon flour, and for a thicker white sauce, add 1 tablespoon flour.