Soak soybeans in a large bowl with lots of water for at least 12 hours, up to 48.
Massage the soaked beans in water and remove as much of the outer hulls as you can. If using split soy beans, skip this step as they have already been de-hulled.
Pour de-hulled soybeans into the Instant Pot and cover with fresh water. Set for 40 minutes at High Pressure and do a natural pressure release.
Drain the cooked soybeans thoroughly and spread onto towels to dry. Leave to dry 30 minutes or use a hairdryer on low to speed things up.
Put dried cooked beans into a bowl, cool to lukewarm (95-98°F). Add vinegar and mix thoroughly.
Mash soybeans with a potato masher until half of the beans are mashed.
Sprinkle tempeh starter over and mix well so it is evenly distributed.
Divide prepared soybeans into two pans that fit inside your Instant Pot. Pack down well with a spatula into a firm cake, about 1 inch thick.
Add 1 cup of water into the Instant Pot and install a trivet. Place the first pan in, install a second trivet or bamboo skewers trimmed to fit over the first pan, then place the second pan on top.
Put the lid on the Instant Pot, vent in any position. Plug in the Instant Pot and hit the Yogurt button three times until it displays "Less." Set the timer for 36 hours.
Let the tempeh incubate for 24-36 hours. Rotate the top and bottom pans to even out mycelium growth. Take care not to drip water from the Instant Pot lid when removing it.
Tempeh is done when it is fully covered with a thick layer of mycelium. Remove from pan and refrigerate up to fours days or freeze for 6°months.
You can pasteurize your tempeh to keep it from continuing to ripen in the refrigerator. This will keep it more sweet and prevent the growth of any other microorganisms that may have gotten into your tempeh. To do this, before removing the tempeh from the pans when you take them out of the Instant Pot, cover them with an oven-safe cover and bake for 30 minutes at 180°F.Cool and refrigerate. This will extend the time you can keep the tempeh in the refrigerator up to 7 to 14 days.