Roast the hazelnuts. Preheat the oven to 350°F/175°C. Place the hazelnuts on a rimmed baking sheet and spread out evenly. Roast in the oven for 10-15 minutes, or until browned and very fragrant.
While the hazelnuts are roasting.1. Line a 8×4-inch (20×10 cm) or 9×5-inch loaf pan (23×13 cm) with parchment paper, letting the excess hang over the long sides to form a sling.2. If the Medjool dates are not very soft, cover them with hot water for 5 minutes, then drain and pat dry.3. Place the oats in the bowl of the food processor and pulverize them until you have a very fine oat flour. Remove the oat flour and set aside.**** Peel the hazelnuts. When the hazelnuts are done roasting, add them to a clean kitchen towel and wrap tightly into a ball. Let them sit wrapped for 1 minute to steam, then rub the towel vigorously to remove the hazelnut skins. It will take a few times, but it’s not necessary to remove all the skins.
Place the hazelnuts in the food processor and blend until you have a a finely ground meal, but don’t blend too long, or it will start to turn into hazelnut butter.
Make the dough. Return the oat flour back to the food processor, along with the pitted dates, cacao powder, salt, vanilla, coconut oil, and maple syrup. Process until you have a sticky dough that you can press together with your fingers. If using cacao nibs, stir them in by hand with a silicone spatula or your hands (remove the blade from the food processor to make this easy, or transfer the dough to a bowl).
Spread the dough out into the prepared pan and smooth the top out evenly, pressing all the way into corners using your fingers or a flat-bottomed steel cup to get the mixture even on top. Place the pan in the freezer for 30 minutes (or longer) to set.
Before removing from the freezer, melt the chocolate.1. Double boiler method: Add a few inches of water to a saucepan and bring to a simmer. Nestle a large heatproof glass bowl that sits on top of the saucepan without touching the water to create a double boiler. Once the water is simmering, add the vegan chocolate chips or chopped dark chocolate to the bowl. Whisk frequently with a spatula until the mixture is just melted.2. Microwave method: Add the chocolate to a microwave-safe bowl and microwave on high in 25 to 30-second intervals until the chocolate is almost melted (about 3 intervals). Use a spoon to stir the chocolate until it is melted. Drizzle the melted chocolate over the cold Nutella dough and lightly sprinkle with flaky sea salt. If the chocolate doesn’t immediately harden, return it to the freezer for 5-10 minutes to set, then slice into 8 thick bars or into smaller squares. Store leftovers in the freezer (they’ll stay good for several weeks).