Soak the pitted dates in boiling water while you prep the hazelnuts.
Toast the hazelnuts in a frying pan over a high heat, shaking the pan often, until the skins start to crack and peel away. Fold the hot hazelnuts onto a clean dish towel and rub together until the majority of the skins have come off. Transfer the hazelnuts with the almonds to a food processor and blend until mealy. Continue to blend, stopping every so often to scrape down the edges of the bowl, until pastey.
Drain the dates then add them in, along with the cocoa, flaxseed, vanilla and salt and blend until smooth. Add maple syrup to taste - start by blending in 2 tbsp, and add more if you want it (you can also sub the additional maple syrup for a few drops of liquid stevia, if preferred).
Press the mixture into a parchment small, parchment-lined loaf tin and freeze for at least 30 minutes. Remove from the tin and cut into around 20 small squares. Store in an airtight container.
Alternatively, you can roll the mixture into teaspoon-sized balls, chill until cool, then coat them in bittersweet chocolate to make truffles.