In a measuring cup, add one cup of regular milk and 1 tablespoon of lemon juice. Stir and let the mixture sit for about 10 minutes.
At the end of that time, you will see the milk begin to curdle.
Use in any recipe as you would buttermilk.
Milk and Vinegar
1 T white vinegar or cider vinegar.
1 C 2% milk or whole milk
Stir vinegar and milk together and let sit for 10 minutes. Use as you would buttermilk.
Other options
For one cup of buttermilk:
*Substitute 1 cup plain Greek yogurt
*3/4 cup sour cream plus enough water to equal 1 cup
*1 cup milk with 1 3/4 tsp cream of tartar. Whisk and let stand until curdled.
*Powdered buttermilk. I keep a canister of this in my refrigerator at all times.
Recipe
If you do buy buttermilk and it expires, don’t throw it away! Buttermilk can last for 1-2 weeks beyond the date on the carton and, if used in cooking or baking, can even be used beyond that point.