Preheat oven to 400 degrees F. Spread the hazelnuts in one layer on a cookie sheet. Roast nuts in preheated oven until dark and aromatic, about 10-15 minutes. Transfer nuts to a damp towel and rub to remove skins. Try to remove as much skin as possible.
Using a food processor, grind hazelnuts until smooth and liquefied, about 5 to 10 minutes. It may take longer, but the nuts will become liquidy if you keep processing.
Add the cocoa, sugar, vanilla, and salt to the hazelnut butter and mix well. While the food processor is running, gradually add oil to reach desired consistency and so it is smooth enough for spreading.
Transfer to an airtight container and store in the refrigerator for up to a month.
Recipe by Wright Family Table at https://www.wrightfamilytable.com/salted-chocolate-hazelnut-butter/