Drain and thoroughly rinse soaked cashews. Add to a blender along with 1 cup water and process until completely smooth.
Transfer to a glass container with a lid. (Ensure that your container is perfectly clean to avoid any unwanted bacteria from growing in your culture.) Add 1 heaping tablespoon vegan yogurt, or the contents of 2 probiotic capsules into cashew cream and stir to combine.
Cover and set in a warm place to culture for 2-4 days. Taste the mixture periodically and move on to the next step when it has reached your desired level of tartness.
The mixture should taste pleasantly tart and be slightly bubbly, but it should not smell funky or change colors in any way. If you observe any strange smells or colors, discard and start your culture again, ensuring that all your containers and utensils are clean.
If you have an Instant Pot, you can speed up this process by using the yogurt function. Pour the cashew cream mixture directly into the pot, or place in the sealed glass container on the rack insert above a few cups of lukewarm water. Incubate for 8-10 hours.