In a saucepan over medium heat, combine all of the ingredients. Whisk vigorously until well-combined.
Bring to a soft boil, then reduce the heat and cover the pan. Let the sauce simmer for about 5 minutes.
Serve immediately or transfer to a mason jar (or similar storage container) and store in the fridge for up to 1 week.
Notes
Mustard:I like to use classic, yellow mustard. You can alternatively use a Dijon mustard or a spicy brown, but I find that the simple, tangy flavor from yellow mustard really is the best option.Brown sugar: You can either use light brown sugar or dark brown sugar, but I prefer light brown sugar to prevent too strong of a molasses flavor in the sauce.Cayenne pepper: If you want to increase the spice level, just double the amount of cayenne pepper. If you are not a fan of spice, cut the amount in half. I don’t recommend omitting it altogether because the cayenne helps to balance the other flavors in the sauce.