Place the hazelnuts in the bowl of a food processor fitted with the blade attachment (or a blender) and mix the machine until fine and resembling cornmeal or fine breadcrumbs. This will take 3 to 5 minutes. Be sure to scrape down the sides and bottom of the bowl so that everything is evenly blended.
Add in the chocolate, brown sugar, butter, vanilla and salt and mix until the hazelnut mixture has formed into a thick, smooth, creamy paste. If it's resisting blending (as can happen in a blender), jump to the next step and add the cream.
With the machine running, slowly drizzle in the heavy cream and continue mixing until a smooth, glossy mixture has formed. I like the hazelnuts a wee on the coarser side for texture, but go as smooth or as coarse as desired. Scoop the mixture into a clean airtight container and store in the fridge until ready to use.