Rub together in a china bowl the butter, vinegar, salt, and dry mustard.
Place the bowl in a saucepan full of boiling water over a spirit lamp, or on the range. Stir the mixture carefully till very hot, to prevent the butter from oiling. When hot add the well-beaten eggs; stir till thick, then pour in the cream, stir, remove from the fire, and allow it to get perfectly cold.
Cold sweet-breads are excellent served with this cream Mayonnaise.