Soak cashews. If you're organised, cover them in water overnight and leave in the fridge. If you're in a rush, boil them for 10 mins.
Drain cashews. Blitz 1/2 a cup of soy milk and cashews in a high speed blender.
Add blended mixture, remaining soy milk and culture/yoghurt to a saucepan. Heat on low, stirring constantly. The aim is to bring the mixture to a lukewarm ~30C. If you don’t have a thermometer, can test the temperature as you go with a couple of droplets on the inside of your wrist.
Transfer to clean glass jars or your yogurt maker container. Cover with lids.
To ferment the yoghurt, we need to keep it at a consistently warm temperature for ~4-6 hours. This gets the bacteria active and gives the yoghurt its tangy flavour. It's important we don't get the yogurt too hot though otherwise it will kill the bacteria. Leave the jar in a sunny spot, or intermittently turn your oven on and off on its lowest temperature. If you have a yoghurt maker, then you don't have to do this - just set it for 4 hours and forget.
After 4 hours, taste test. It should have a mild tang, and a smooth, thick pot-set texture. You can make it more tangy by leaving it for an extra 1-2 hours. Don't go over this time as it may cause the yoghurt to split.
Chill the yogurt in the fridge for a few hours, it will get even thicker.
If desired, stir through a teaspoon of vanilla and syrup to taste.