Whisk together vinegar and dry mustard powder in a non-reactive bowl. Cover mustard mixture and leave overnight. In a small pot combine the mustard mixture and sugar. Use a thermometer to watch the temperature. Add 1 large egg to the mustard sugar mixture and whisk to combine. Heat the mixture until it reaches 165 degrees, and heat for a couple of minutes. Remove from stove. Store sweet hot mustard in the refrigerator for up to 4 weeks after it is made.