Preheat the oven to 350°F (176°C) and place the hazelnuts on a rimmed baking sheet. There should only be a single layer. Use more baking sheets, if necessary.

Roast the nuts for 15 minutes, stirring after every 5 minutes. The hazelnuts will be fragrant when ready and the skins will start to crack. Place the nuts in the center of a spread-out clean kitchen towel. Bundle it up and then rub the nuts together to remove the skin. This should remove about half of the skin and that's enough. The nuts should still be warm but not super hot at this point. If they are hot, wait for a few minutes, until they're just warm, before placing in the food processor. Also be sure not to let the nuts cool down completely – it'll process quicker if the nuts are still warm.

Transfer the roasted nuts, being careful to not include the skin, to a food processor and process until it’s very creamy like peanut butter. It’ll depend on your food processor, but it took mine about 3–5 minutes.

Add 3 tablespoons of cocoa powder, 1/4 cup sugar, and the salt. Add more cocoa powder and sugar to taste.

Spoon the butter into a glass jar and refrigerate for up to 2 months.