Preheat the oven to 285°F (140°C) fan assisted or 320°F (160°C) conventional.
To make the filling: Spread the hazelnuts on a baking tray and roast for 40 to 50 minutes, until lightly golden. Remove from the oven and let cool for a few minutes.
Place 1 cup (136 g/4.8 oz) of the roasted hazelnuts in a food processor. Process for 1 to 2 minutes, until chunky. Add the coconut butter, butter, collagen powder, cacao powder, vanilla, and sweetener, if using. Process again until well combined. Place the dough in the fridge to set for 1 hour.
Reserve 12 hazelnuts for filling and crumble the remaining hazelnuts into small pieces.
To make the chocolate coating: Line a baking tray with parchment. Melt the dark chocolate and cacao butter in a double boiler, or use a heatproof bowl placed over a small saucepan filled with 1 cup (240 ml) of water, placed over medium heat. Remove from the heat and let cool to room temperature before using for coating. Alternatively, use a microwave and melt in short 10- to 15-second bursts until melted, stirring in between.
Remove the dough from the fridge and use a spoon to scoop about 1 ounce (28 g) of the dough. Press one whole hazelnut into the center and use your hands to wrap the dough around to create a truffle. Place in the freezer for about 15 minutes.
Gently pierce each very cold truffle with a toothpick or a fork. Working one at a time, hold the truffle over the melted chocolate and spoon the chocolate over it to coat completely. Turn the toothpick as you work until the coating is solidified. Place the coated truffles on the lined tray and drizzle any remaining coating over them. Before they become completely solid, roll them in the chopped nuts. Refrigerate the coated truffles for at least 15 minutes to harden.