Mix all the dry ingredients in a microwave safe bowl and stir with a whisk. In a separate bowl, combine the beaten egg, milk, and butter. Combine the wet ingredients to the bowl with dry and whisk. Add vinegar. Heat the mixture in the microwave on high for 2 minutes and whisk until smooth. Return to microwave and heat for 2 more minutes, then remove and whisk until smooth. Microwave for one additional minute and whisk smooth. Place the microwavable bowl in a bowl full of ice to cool temperature rapidly. Whisk the mustard every few minutes until cool. Refrigerate for up to 2 weeks.
Serve with pretzels or crackers.