Preheat your oven to 375°F; spread the hazelnuts onto a large baking sheet and put them in the preheated oven for 10-12 minutes.
Remove the toasted nuts from the oven and let them cool for a few minutes, until you can safely handle them with your bare hands.
Transfer the warm nuts to a clean tea towel and bring all 4 corners of the towel together to form a little bundle. Hold that bundle in one hand and, with the other hand, wiggle the nuts around for a few minutes so that they grind against each other. The friction will cause the skins to fall off the nuts.
Transfer the nuts to the bowl of your food processor, being careful to leave all the skins behind. Add the salt and process on high speed, scraping the bowl from time to time, until the nuts turn into a creamy butter, about 5 minutes total.
Transfer about half of this nut butter to an 8oz glass jar with a tight fitting lid. Mason jars work beautifully!*
Add cacao powder and coconut sugar to the remaining hazelnut butter and process until well incorporated, about 30 seconds. You might want to scrape the sides once in the process to make sure the mix is well combined.
Transfer to another 8oz glass jar with a tight fitting lid.
Let your nut butters rest until the next day to allow their flavor to fully develop. They will keep for several weeks at room temperature or several months in the refrigerator.
*If you wanted to make the whole batch dark chocolate, simply double the amount of cocoa powder and coconut sugar and add them to the whole batch of hazelnut butter once it has reached a smooth consistency.