Add the lemon juice or vinegar to a measuring cup.
Pour in the milk up to the 1-cup (240 milliliters) line. You can use it immediately, but ideally, let the mixture sit for 5-10 minutes to allow the milk to slightly curdle, giving it a texture and taste similar to buttermilk. Use it as a 1:1 substitute for buttermilk in most recipes, including baking.
Video
Notes
Store in an airtight container in the fridge for 3-5 days, shaking well before use, as it may separate.
For freezing, pour into ice cube trays or a freezer-safe container, then transfer to a resealable bag. Thaw in the fridge or at room temperature, and stir before using.