In a food processor, add hazelnuts. Blend them until they look like coarse flour, about a minute.
Add cacao powder, sugar, vanilla, and salt. Blend for about 20 minutes, stopping every 5 minutes to scrape the sides so everything gets incorporated. The longer you blend the more liquid it will become.
When it's ready, transfer it to a jar but don't close it until it's completely cooled. Store in your pantry or refrigerate.