Whisk together dry ingredients in a microwave-safe bowl. In a separate bowl, combine beaten egg, milk and butter. Add wet ingredients to bowl with dry ingredients and whisk. Add vinegar. Heat mixture in the microwave on high for 2 min. and whisk until smooth. Return to microwave and heat for 2 more min., then remove and whisk until smooth. Microwave for 1 more min. and whisk until smooth. Set bowl in another bowl full of ice to cool temperature rapidly. Whisk the mustard every few minutes until cool. Refrigerate in clean, sealed jar(s) for up to 2 weeks.
Serve with pretzels or crackers.