Preheat the oven to 350°F. Lightly grease a 9-inch springform pan with softened butter or nonstick cooking spray. (You can use any 9-inch cake pan, but line it with parchment paper with some overhang so you can easily remove the finished cake.) Sprinkle the inside with sugar and rotate the pan to coat the bottom and sides evenly; tap out excess.
Make the cake. Combine the chocolate, Nutella, and butter in a large heatproof bowl. Set the bowl over a small pot of simmering water and heat, stirring often, until the chocolate and butter have melted and you can stir everything together to a smooth, creamy mixture. Remove from the heat and set aside. (Alternatively, microwave in 30-second increments until evenly melted.)
Separate 4 of the eggs, placing the whites in a large mixing bowl (either a bowl fitted for a stand mixer or a bowl large enough to handle a hand mixer). Place the yolks in another large bowl and add the hazelnut flour, cocoa powder, salt, and the remaining 2 whole eggs and whisk to blend well. Using a spatula, gently and gradually mix the egg yolk mixture into the melted chocolate mixture (don’t use a whisk here; the batter is quite thick and will get stuck in the wires).
With the mixer on high, beat the egg whites. When they start to get light and foamy, gradually add ¾ cup sugar, a tablespoon or two at a time, and continue to beat until egg whites have tripled in volume and are light, fluffy, opaque, and hold stiff peaks. They should look like a very good meringue that you could frost a cake with (that’s not what you’ll be doing, but just saying).
Using a spatula, gently fold the egg whites into the chocolate mixture until just combined and no obvious white streaks remain (this will look cool—maybe take a picture!); avoid overmixing (that would deflate all that air you worked so hard to build into those egg whites).
Pour the batter into the prepared pan and smooth the top. Sprinkle with the remaining 2 tablespoons granulated sugar and bake until the edges begin to pull away from the sides of the pan and the top looks puffed and lightly cracked, like a soufflé (it should still have a little jiggle), 35 to 40 minutes.
Let cool completely (if you have a wire rack, use it). During this time, something seemingly tragic will happen—the center of the cake will collapse, causing further cracking around the edges. This is the intended effect, so don’t worry—it’s where those crispy edges come from, the reason we are all here.
Prepare the topping/accompaniment. Using an electric mixer (or a good old-fashioned whisk and elbow grease), whip the cream, confectioners’ sugar, and salt in a medium bowl until you’ve got medium-stiff peaks, then whisk in the sour cream. For a streaky effect, fold in the Nutella using a spatula or if, you know, who cares, just whisk it in. Use this mixture to top the cake, but I prefer to eat it on the side (so as to not ruin the cake’s crispy texture) with some delicious cherries for snacking on in between bites.
Do Ahead: You can bake this cake up to 2 days ahead, wrap it tightly, and store at room temperature.
Note: Using an unsweetened spread like almond butter or tahini will give you a slightly less sweet version of this cake, which for my taste, is still perfectly sweet enough.
Alison Roman is a cook, writer and author of the bestselling cookbook "Dining In," published by Clarkson Potter in Fall 2017. Her second cookbook "Nothing Fancy," is now available. She is a bi-weekly columnist for the New York Times Cooking section, as well as a monthly contributor to Bon Appétit Magazine. Originally from Los Angeles, she lives in Brooklyn until she decides to move upstate like everyone else.