What's the substitute for buttermilk powder?

1/4 cup powder + 1 cup water. OR Use 1 cup sour cream OR For baking use 1 cup milk + 1 1/2 teaspoons cream of tartar OR - For 1 cup buttermilk you can combine 1/3 cup dry powdered milk + 1 cup of water, then add 1 tablespoon vinegar or lemon juice. Allow mixture to sit for 5 minutes before using