Let the butter get hot in a sauce-pan. Add the flour, and stir until smooth, being careful not to brown. Add the milk, and boil up. Place the sauce-pan in another of hot water.
Beat the eggs, salt, pepper, sugar and mustard together, and add the vinegar. Stir this into the boiling mixture, and stir until it thickens like soft custard, which will be in about fire minutes.
Set away to cool; and when cold, bottle, and place in the ice-chest.
This will keep two weeks.