Making sour milk, in place of buttermilk

1 cup whole or 2% milk
1 teaspoon of vinegar or lemon juice

Mix them together and let sit at room temperature for 10-15 minutes. It will start to curdle just a little bit. This is not the same as milk that has been left out to rot. Buttermilk or sour milk is used in many cookies, breads and pies and creates a much different flavor in the outcome.