1/2 tsp. salt
1 tsp. mustard (dry)
1 tbsp. flour
2/3 c. sugar
1 tbsp. melted butter
2 eggs, beaten
1/2 c. vinegar
1/2 c. water

Put first 6 ingredients in a double boiler. Beat over medium heat until fluffy and sugar is dissolved. Add vinegar and water and continue beating until mixture begins to thicken. (Mixture will thicken more as it cools.)

NOTE: Can also be done in microwave. Beat ingredients, then heat 1 minute at a time, stirring between until thick.