Place the chickpeas in a large bowl, cover with clean water and soak overnight.
The next day, drain and rinse the chickpeas. Transfer to a large cooking pot, cover with water (about 2-inch higher than the chickpeas). Bring to a boil and let simmer for 1 hour. Depending on the size of your chickpeas it might take up to 1 and ½ hour. They must be soft but not mushy. If some foam forms on the surface during cooking, remove it with a spoon to prevent any spillover.
Once the chickpeas are cooked, drain them and leave them in the strainer for 30 minutes. After that, pat them dry using a paper towel to make sure they are mostly dry on the outside.
Transfer to a large clean bowl. Add the white vinegar and stir to coat. Add the tempeh starter and mix well using a spoon until uniformly distributed.
Using a metal skewer, or chopstick, prick some holes (at about 2-inch intervals) in a clean freezer bag. This step is important to create good air circulation and allow the mold to grow.
Transfer the chickpeas to the freezer bag and form a rectangle of about 5x8 inches with a thickness of about 1-inch. Place the bag of chickpeas on a baking sheet and put in an oven with the light on for about 14 hours. I set the temperature of my oven to 86°F and let the door very slightly opened because I can't just let the light on.
After 14 hours, some white mold (not a lot) should have appeared on the chickpeas. You might also see some condensation inside the plastic bag, this is normal. It's time to remove the baking sheet from the oven and let it ferment for another 24-36H in a dark and warm place. I recommend covering the chickpea bag with a clean towel to make sure it's not under direct sunlight.
Your tempeh is ready when it is fully covered with white mold and forms a solid cake, which usually happens at the 48-hour mark. It can be quicker if the temperature in your house is high.
Raw tempeh will keep for up to a week in the refrigerator. If you want to freeze it: steam the tempeh for 25 minutes, let cool completely and wrap in plastic film before freezing.