Whisk together all the ingredients for the ice cream in a bowl. If the cashew butter or tahini have clumps, melt them before using.
Once a smooth mixture has formed, pour the mixture into a shallow, airtight container.
Place in the freezer, with the lid on. Stir it every hour, until it reaches the desired texture. This will take around 5-6 hours.
In the meantime, prepare the hazelnuts. Heat a skillet over medium heat, and add in the hazelnuts. Do not chop them - keep them whole.
Stir them around for just a few minutes, until you can smell the aroma. Taste test a small piece - if you can taste that "hazelnut" flavor coming through stronger than the raw nuts, then turn the heat off and set them aside. They will burn very quickly, so keep a close eye on them!
Once they are toasted, add the nuts to a clean kitchen towel. Rub them together with the towel to remove the skins.
When most of the skins have been removed, chop up the nuts. Then set them aside.
Once the ice cream has achieved the desired texture, remove from the freezer.
Gently fold in the chopped hazelnuts and chocolate. Serve right away, or place in the freezer and thaw to the desired consistency before serving.