of acid.
gently stir, then let it rest for 10 minutes at room temperature.
stir again. use immediately or store in the refrigerator for up to a week.
i haven’t found that the extra tablespoon of liquid makes a difference in the recipes where i use homemade buttermilk, but if it’s a concern for you, measure the 1 tablespoon of acid into your measuring cup first, then pour in the milk to a level 1 cup. 