Drop the yolks into a small enamelled stewpan, add the pepper and salt, and stir well with a wooden spoon; pour in the milk, which should be just at boiling point, then stir briskly over a gentle heat for about ten minutes, or until the sauce thickens, but it must on no account be allowed to boil, or it will curdle. When sufficiently thick, remove from the fire, stir in the vinegar, and stand on one side to get thoroughly cold. It is then ready for use.