A common ingredient in vintage recipes is “sour milk.” Especially during the Great Depression, waste was avoided, and a cake was a good use of milk that had soured or was close to. Instead of having to wait around for your milk to spoil, there is a quick shortcut to sour milk:

1 cup whole milk

1 Tablespoon lemon juice or white vinegar

Allow to sit for at least 5 minutes

Buttermilk can be substituted for sour milk (omit the lemon juice/vinegar) and will give your recipe a richer flavor. Sour milk can also be substituted for buttermilk!