Special equipment: 9- by 3-inch springform pan
Preheat the oven to 375 degrees F. Prepare the springform pan by coating it in a thin layer with 2 tablespoons of the softened butter. Add the cocoa powder to the pan and tap the sides to coat the bottom and sides evenly. Discard any excess cocoa powder. Put the hazelnuts on a baking sheet and toast in the oven on the center rack until just fragrant, 8 to 10 minutes. Transfer immediately to a kitchen towel to stop them from cooking. Use the towel to rub off the skins. Set aside to cool. Lower the oven to 350 degrees F.
Fill a 1- to 2-quart pot with about 1 inch of water and bring it to a bare simmer over medium heat. Place the dark chocolate pieces into a heatproof bowl and set it over the pot and simmering water (this is a bain-marie). The water should not touch the bottom of the bowl. Let the chocolate melt, and then stir gently. Remove from the heat when melted.
Chop the hazelnuts into small pieces using a chef's knife or a food processor. Make sure you don't turn them into powder. Whip the egg whites with an electric mixer with the salt until they are almost at stiff peaks. Try not to over-whip. Mix together the remaining 5 tablespoons butter with the chocolate hazelnut spread in a large bowl with a wooden spoon. Stir in the espresso. Add the yolks and stir briskly to combine. Stir in the chopped hazelnuts and melted chocolate. Add one-fourth of the whipped egg whites to the chocolate hazelnut mixture and stir to lighten the batter. Then gently fold the rest of the egg whites into the batter in 3 additions. Don't over-fold. Pour the batter into the prepared cake pan.
Bake on the center rack of the oven until the sides of the cake start releasing from edges of the pan, 40 minutes. This cake is almost a hazelnut fudge, so no need to poke with a toothpick to check if the inside is ready or not. The cake will come out of the oven nice and fluffy and souffled, but it will deflate to half the size during its cool-off period. Serve garnished with some powdered sugar and whipped or cream or gelato, if using.
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