Preheat oven to 350 degrees and spread the hazelnuts in a single layer on a rimmed baking sheet. Toast in the oven until the nuts have darkened and the skins have blistered, about 12-14 minutes. Remove from oven and let cool slightly. Wrap the nuts in a kitchen towel and rub to remove as much of the skin as possible. Don't worry about any of the stubborn skin that clings to the hazelnut. Let nuts cool completely.
Melt the chocolate in a double boiler until almost melted. Turn off heat and stir until smooth. Set aside and let cool slightly.
In a high-speed blender (I use a Vitamix) or a food processor, grind the hazelnuts until they form a paste. Add the oil, powdered sugar, cocoa powder, vanilla, and salt. Continue to process the mixture until the mixture is as smooth as possible then add the melted chocolate and continue to blend until very smooth and well combined.
The homemade nutella may be thin but it will thicken as it cools. If the mixture is already pretty thick add a bit of extra oil to keep the mixture more spreadable after it cools.
Transfer the spread into a jar or other resealable container and cool to room temperature. Cover and store at room temperature or in the refrigerator. If you store it in the refrigerator you will need to bring to room temperature in order for it to be at a spreadable consistency.