Drain and rinse the soaked cashew nuts and add them to a blender.
Pour the rest of the ingredients into the blender and blend on high for about 5 minutes until the mixture is completely smooth and silky. No lumps should remain.
Transfer the mixture into a bowl and cover with a clean towel, tea towel or muslin cloth.
Keep in a warm place (like an airing cupboard) for 24-28 hours, depending on how tangy you want it! It will be ready when the mixture looks foamy and is coming away from the sides of the bowls. It will also have a strong but pleasant tangy smell!
When the yoghurt has fermented give it a good stir or mix again in a clean blender if needed. Transfer to a jar or container and keep in the fridge for up to 1 week.
To make into frozen yoghurt:
If using added flavourings (like a probiotic capsule, pureed fruit, lemon zest or cocoa powder), stir in or blend with the yoghurt until well combined.
Pour the mixture into a shallow freezer safe container and spread it out evenly. Secure the lid and place it in the freezer for an hour.
After an hour, take the mixture out of the freezer and give it a good stir to break up any icy bits that may have formed and make the mixture nice and creamy.
Place back in the freezer and allow to freeze completely (for at least another hour or overnight).
To serve, take the container out of the freezer and allow to sit at room temperature for 1 minute so it softens slightly. Scoop and serve!