Grease and line a 9-10 in. (23-25 cm) springform cake pan with parchment paper. (If you don't have a springform pan, you can use a regular cake pan, but it will be more difficult to remove the cake). Preheat the oven to 325° F (163° C).
Place the hazelnuts in a food processor (or high-powered blender) and pulse until the hazelnuts are finely ground. Scrape the sides a couple times to ensure the flour is evenly ground. Make sure not to pulse it too much, or it will turn into hazelnut butter! Set aside.
Break the chocolate into pieces, and place it in a medium-sized, heat-proof bowl with the unsalted butter. Set the bowl over a pot of simmering water, and stir constantly with a wood spoon until the chocolate-butter mixture is completely melted and smooth. Don't overheat, or the chocolate may seize.
Remove the bowl from the heat, add the sugar and hazelnut flour to the chocolate-butter mixture, and stir until completely mixed. Ensure the mixture is mixed very well or the butter may separate in the oven! Set aside to cool to room-temperature.
Separate the egg yolks from the whites. In a small bowl, whip the egg whites until stiff and shiny (but not dry). Add the egg yolks to the chocolate hazelnut batter, one at a time, until completely mixed. Stir in the vanilla and salt. Then, gently fold the egg whites into the chocolate hazelnut batter until no longer streaky.
Pour the chocolate hazelnut batter into the prepared pan, and smooth the top. Place the pear halves on top (or chop the pears and sprinkle them over the top). Bake for 30-35 minutes, until the top is dry and a cake tester inserted in the center comes out with just a few crumbs. The pears may make the cake appear under-baked, but be careful not to cook it for too long, or it will get dry. 35 minutes was perfect for my oven.
Let the cake cool completely in the pan. Remove the cake from the pan, and sprinkle with powdered sugar. Serve with maple cream and cinnamon.