1Tablespoonlemon juice (fresh or bottled)may substitute 1 Tablespoon white distilled vinegar
1scant cup*milk
Instructions
Pour one Tablespoon of lemon juice (or one tablespoon of vinegar) into a liquid measuring cup.
Add milk until liquid reaches the 1-cup line (at eye level).
Stir. The milk will thicken a little and appear to be slightly curdled or chunky. For peace of mind, let it sit for 5 minutes, even though we learned above that this isn't truly necessary.
Use buttermilk in your recipe as instructed.
Video
Notes
*A scant cup just means just shy (usually 1-2 Tablespoons) of a full cup. In this instance, we're pouring the lemon juice into the measuring cup first, so, even though we're filling the milk up to the 1-cup line, the amount of actual milk that is used will be just less than a full cup, or a "scant cup".