In a small bowl, combine mustard and vinegar. Cover and let stand at room temperature for 8 hours or overnight.
In a large saucepan, whisk the eggs, sugar, molasses, honey and mustard mixture. Cook and stir over low heat until mixture is thickened and a thermometer reads 160°, about 20 minutes. Cool. Stir in mayonnaise and mustard seed if desired. Cover and refrigerate for up to 3 weeks.