Toast the hazelnuts in a single layer on a baking sheet, until they've slightly browned, about 10-15 minutes. Stir once or twice while they're in there.
Wrap them in a clean towel and rub to remove lose skin. (Skip this step if you were lucky enough to find them with the skins already removed.) Set aside to let cool completely.
Melt the chocolate in either the microwave or a double boiler. Let cool but try not to let it harden again.
Grind the hazelnuts in a food processor or blender until they form a nut butter. Add the oil, cocoa powder, vanilla, and salt. Blend again to get as smooth as you'd like it. (Generally the smoother the better, unless you're a chunky peanut butter fan.)
Add the melted chocolate and blend well to combine. If desired, strain the mixture.
Pour into a jar or other sealable container and let cool at room temperature with the lid off. Cover and store at room temp for up to 2 weeks.