Add the milk to a jar.
Pour in a tablespoon of lemon juice or vinegar.
Stir well.
Set aside for 10 minutes.
If using for recipes where consistency is crucial (e.g., buttermilk chicken or salad dressing), transfer to a sealable jar.
Shake well, then set aside for another 10 minutes.
Shake once more before using!

Notes:
– You can keep the buttermilk replacement in the fridge for up to a week or freeze it for up to two months.
– For a non-dairy version, substitute the milk with coconut milk.
– You can also use 3/4 cup sour cream or yogurt thinned out with 1/4 cup of water as a buttermilk replacement.
– If you don’t have lemon juice or vinegar on hand, you can mix 1 3/4 tablespoons of cream of tartar into the cup of milk, then follow the same steps above.