Start by preheating the oven to 175C and line a large baking tray with parchment paper. Add the hazelnuts in a single layer and roast for 10-15 minutes until golden (do NOT let them burn)
Let them to cool for a few minutes and add them to a clean tea towel and rub to remove most of the skins.
Add to your food processor and blend until they turn into nut butter (you will need to stop to scrape down the sides a couple of times) - this should take around 5 minutes.
Once smooth (mostly - there will likely still be some lumps), add in the cocoa powder, icing sugar, olive oil and generous pinch of salt and blend again until thoroughly combined and a delicious chocolate-y nut butter has formed. You can use liquid sweeteners if you would prefer, but beware that this will cause the nut butter to 'seize up' and have a different texture. Taste and add any more ingredients to taste and store in an airtight jar or container.