On rimmed baking sheet, bake hazelnuts at 350° until toasted, about 15 minutes. Wrap hazelnuts in towel; rub vigorously to remove skins. In food processor, finely grind hazelnuts, occasionally scraping down sides of bowl, about 3 minutes. Add oil; blend for 2 minutes. Add chocolate, sugar, cocoa, and 1/4 tsp. salt; blend until smooth, about 1 minute. Strain through fine-mesh sieve. Cover and refrigerate to store.