Position the oven rack in the center and preheat your oven to 400°F (200°C). Lightly grease an 8-inch round cheesecake pan with butter and set it aside. A springform pan of the same size works just as well.

Melt the chocolate using a double boiler method. Fill a small saucepan with water and bring it to a boil. Once boiling, lower the heat and place a heatproof bowl over the pot, making sure it sits securely without touching the water. Add the chopped chocolate to the bowl and stir continuously until fully melted. Remove the bowl from heat and allow the chocolate to cool completely before using.

Spread the hazelnuts evenly on a rimmed baking sheet. Once the oven has reached temperature, place the baking sheet inside and roast the nuts until golden brown, about 6 minutes. Remove from the oven and let them cool completely. Reduce the oven temperature to 350°F (180°C).

To remove the hazelnut skins, rub the cooled nuts together using a clean, coarse kitchen towel. Transfer the peeled nuts to a food processor and pulse 10 to 12 times, or until they resemble a coarse sand texture. Avoid over-processing to prevent releasing excess oils. Set the ground hazelnuts aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter and sugar on medium speed for about 2 minutes, until light and fluffy. Add the eggs and vanilla extract, and mix until fully combined. Add the flour, salt, and cinnamon, and continue beating for another 30 seconds. Then, add the ground hazelnuts and cooled melted chocolate, and beat for about 1 minute until everything is well incorporated.

Transfer the batter to the prepared pan, smoothing the top evenly. Bake for approximately 30 minutes, or until a toothpick inserted in the center comes out with just a few matte-looking crumbs. The center may appear slightly underdone, but as long as the toothpick isn't coated in wet batter, the cake is ready. Remove from the oven and let the cake cool in the pan on a wire rack.

Once fully cooled, gently release the sides of the springform or cheesecake pan. Invert the cake onto a serving plate and carefully peel off the parchment paper from the bottom. Let the cake rest for about 20 minutes, then generously dust the top with powdered sugar for garnish. Serve right away or wrap tightly in plastic wrap and store in the refrigerator.