1/2 to 1 cup cultured buttermilk (from store or elsewhere)
Enough milk to fill a quart sized jar
Place cultured buttermilk in jar. Add enough milk to fill the rest of the jar. Place lid tightly onto the jar. Shake. Place in a warm area - away from sunlight and let sit for 12-36 hours.
Refrigerate once it has thickened some and has a tart flavor. Use within 2 -3 weeks. You can use this as a starter for your next batch within that 2-3 week period.
*You can make your own buttermilk starter by following these steps:
1. Allow one cup of raw milk to sit covered at room temp. until it has clabbered (when the milk has fermented). This will take several days.
2. Place 1/4 cup of the clabbered milk in a pint mason jar. Add one cup of fresh milk, cover and shake to mix. Allow to sit at room temp. until clabbered.
3. Repeat these two steps until the milk independently clabbers after 24 hours. Taste should be tart, texture should be thickened, and has no "off" flavors.
4. Use this milk culture as your buttermilk starter and follow buttermilk recipe.