Homemade buttermilk is a breeze to make and freezes well so you'll never run out again!
Add cultured buttermilk and the milk to the jar.
Stir together and cover with the lid.
Place in a warm, dark place (like a kitchen cupboard or on top of the fridge) for 12-24 hours.
Put the jar of freshly cultured buttermilk into the fridge.
Make a new batch weekly to keep the culture strong.
You can make buttermilk with either low-fat or whole milk. I prefer whole milk because it results in a thicker and more flavorful culture.