Soak the nuts overnight OR by covering with boiling water and letting sit for 30 minutes until softened.
Drain and rinse the nuts, then puree with 2 cups of water and date for optional sweetness. Strain using a nut milk bag. If you don't have a nut milk bag, you can use a fine mesh strainer, though note that the nut milk bag will produce a much smoother liquid.
Place the strained milk and cornstarch or arrowroot powder in a medium saucepan and whisk very well, then heat over medium heat for 5-7 minutes, whisking often, until yogurt has thickened.
Remove from heat and let cool completely, then whisk again (a film will form on top as the yogurt cools. You can reduce this by placing a piece of plastic wrap over the top to reduce any film, or just whisk it in once cooled.
Add in prepared non-dairy yogurt to help culture. Then cover and cook using either the oven or Instant Pot.
For the oven: Preheat oven at 200F for 15 minutes, then turn off the oven and keep the light on. Place the yogurt mixture for 8-9 hours to culture.
For the Instant Pot: Place steam rack in Instant pot and covered bowl of yogurt on top. Process for 8 hours.
Whisk to remove any lumps.
Enjoy as is, or stir in chopped fruit, vanilla, etc.