Start by toasting the nuts. Preheat oven to 350° F. Place hazelnuts in a single layer on a shallow baking pan. Toast until the skins are almost black and the meat is dark brown, about 15 minutes. (To get rid of the bitter skins, wrap the cooled hazelnuts in a clean kitchen towel or paper towel. Rub until most of the skins come off, but don’t worry if some remain. If you buy them skinless then just toast them.)

Process nuts in a food processor, scraping down the sides of the bowl occasionally, until they look like peanut butter, about 5 minutes.

In the meantime, place chocolate in bowl with oil and microwave for about 1 minute. Remove and stir until fully mixed and smooth. 

Add the agave, vanilla and salt to the chocolate and pour the whole thing into the food processor. Process until smooth. If it is not as spreadable as you prefer then add more oil to reach desired consistency.

Transfer the spread to an airtight container, and store in the refrigerator for 1-2 months. For best results, stir the chocolate-hazelnut spread before using.