In a medium saucepan, whisk together the brownie mix and milk over medium heat and cook until smooth and thick, about 5 to 8 minutes. There shouldn’t be any grittiness to the mixture, it should look like chocolate sauce.
Storage Tip: Brownie Batter Dip is best within 2 to 3 days but lasts up to a week in the refrigerator when stored in an airtight container. But with a dip this good I wouldn’t count on there being much for leftovers!