I swear every time I am half way through making my recipe is when I finally realize I don't have any buttermilk! So I always use this trick, and my recipe still turns out perfect.
Almost 1 cup milk
1 tblsp vinegar or lemon juice
Place vinegar or lemon juice in a measuring cup. Add enough milk to make 1 cup. Let stand for a few minutes. Use as much buttermilk as your recipe needs.
Makes 8 ounces