In a metal bowl or the top of a double boiler, whisk together the malt vinegar and mustard powder until smooth. Cover with plastic wrap and let sit overnight on the counter.
The next day
Sterilize the half-pint jars and lids by running them through the dishwasher or boiling them for ten minutes. Set aside.
Bring 2-3 inches of water to a boil in the bottom of the double boiler or pot of your choice. Ensure the bowl does not touch the water.
Stir the sugar into the mustard mixture, then add the eggs. Mix until well combined.
Place the metal bowl over the boiling water. Cook the mixture, stirring constantly, for about 10-20 minutes until it thickens to about the consistency of Greek yogurt. Remove from heat.
Ladle the mustard into the prepared jars, clean the rims with a wet towel, and tighten the lids. Store in the fridge. The mustard will keep for 2-3 months in the refrigerator.
Notes
This recipe doubles well; a double batch makes 7 half-pint jars.
I exclusively use Coleman’s mustard powder; using a different brand may impact the flavor.
You can adjust the cook time to achieve your preferred consistency. I like it on the thicker side.