01 - Preheat oven to 350°F. Spread hazelnuts in a single layer on a baking sheet and roast for 8 to 10 minutes, until fragrant and skins are cracking.
02 - Take hazelnuts out of the oven and transfer to a clean kitchen towel. Wrap and rub them vigorously to remove most of the skins, leaving some skin as needed.
03 - Place warm hazelnuts in a food processor and blend until a smooth and creamy butter is formed, scraping down the sides periodically. This process should take about 4 to 5 minutes.
04 - While the nuts are processing, melt the chopped dark chocolate in a heatproof bowl set over a pan of simmering water, stirring until fully smooth. Remove from heat and let cool slightly.
05 - Add melted chocolate, cocoa powder, sugar, vanilla extract, neutral oil, and sea salt to the hazelnut butter and process until smooth and spreadable, about 2 to 3 minutes. Scrape down the sides as needed.
06 - Taste and adjust sweetness or salt to your preference.
07 - Transfer the spread to a clean jar and allow it to cool completely before sealing. Store at room temperature for up to 2 weeks, or refrigerate for a longer shelf life. The spread will firm up when chilled.