Matsoni is a traditional yogurt popular in the Caucusus and Armenia in particular. It is mildly tart and creamy with a semi-solid, syrupy consistency. It requires only two ingredients: starter culture and milk.
Whisk matsoni starter culture with whole milk in a medium bowl, and pour into a quart-sized jar. Cover loosely and place it in a warm spot in your kitchen where it will culture for 24 to 48 hours.
When the milk forms a semi-solid mass and pulls away from the sides of the jar when you tilt it, the matsoni is ready. Transfer it to the refrigerator to halt fermentation. Serve as you would any other yogurt. Reserve ¼ cup to culture another batch.