Place eggs in top of double boiler over gently simmering water and whisk until light.
Add sugar, mustard, salt and pepper and blend well.
Gradually add cream, stirring constantly with wooden spoon until it drops in thin ribbon, about 8 to 10 minutes.
Do not boil or mixture will curdle.
Remove from heat and slowly add vinegar, beating constantly to blend.
Mixture will thicken as it cools.
Store in refrigerator