Preheat the oven to 350 degrees.

Spread the hazelnuts in a single layer on a baking sheet and toast them in the oven until fragrant and golden, 10 to 15 minutes. Wrap the warm nuts in a clean kitchen towel and rub them vigorously to remove as much of the loose skins as possible. Don't worry if all the skins don't come off completely. (You can skip this last step, of course, if you've found pre-skinned hazelnuts.)

In a food processor (or Vitamix blender), process the still-warm, toasted hazelnuts until they form a paste, about 5 minutes, scraping down the sides of the bowl with a rubber spatula as necessary.

Meanwhile, melt the chocolate in a metal bowl placed over a saucepan of gently simmering water, or in the microwave at low power. Stir until smooth.

Add 1 tablespoon of oil, the powdered sugar, cocoa powder, vanilla, and salt to the ground nuts in the food processor and continue processing until the mixture is as smooth as possible. Add the melted chocolate and process again until well blended. If it is too thick and stiff, add another tablespoon of oil.

The mixture may be a bit thin at this point, but will thicken as it cools. Transfer the mixture to a jar or resealable container and refrigerate for several hours until it firms up.

It should keep in an airtight container for up to 1 month in your refrigerator.

If it gets too firm and you need to soften it for easy spreading, microwave it for about 5 seconds.