Pour 1 cup of unsweetened, unflavored soy milk into a bowl or jar. Remove one tablespoon of the milk.
Add 1 tablespoon cider vinegar. Whisk to combine.
Let the mixture rest, uncovered, for 10 minutes. It is ready when thickened and lumpy.
Keep leftover buttermilk refrigerated for 3-4 days or freeze for up to 3 months.
Notes
Milk: Soy milk works best due to its high protein content. Oat milk and pea protein milk may also work, but I've had inconsistent results so I don't recommend using that as your first option. I don't recommend almond, rice, or coconut milk.
Vinegar: This recipe works with an equal amount of plain vinegar or lemon juice.