Step 1

Place a rack in middle of oven; preheat to 325°. Using your fingertips, generously butter bottom and sides of a 9" round cake pan (make sure pan is at least 2" deep). Line bottom of pan with parchment paper, smoothing to eliminate air bubbles. (It may sound fancy, but making a parchment circle is easy! Place pan on a piece of parchment, trace circumference with a pencil, then cut it out.)

Step 2

Coarsely chop 12 oz. good-quality bittersweet chocolate into ½" pieces. Transfer to a medium saucepan.

Step 3

Add ¾ cup (1½ sticks) unsalted butter to saucepan and cook over low heat, stirring occasionally with a heatproof spatula, until butter and chocolate are melted, about 8 minutes. (Alternatively, microwave chocolate and butter in a heatproof bowl in short bursts, stirring in between, until melted and homogenous.)

Step 4

Remove pan from heat and let cool to room temperature, about 15 minutes. (You can encourage chocolate mixture to cool faster by occasionally stirring it.)

Step 5

Meanwhile, using an electric mixer on high speed, beat 6 large eggs, 1¼ cups granulated sugar, 1 tsp. kosher salt, and 1 tsp. vanilla in a large bowl until pale yellow and fluffy, 8–11 minutes. You’ll know it’s ready when the egg mixture has almost tripled in volume, the beaters start to create a ribboning effect, and when you lift the beaters, the mixture should leave a trail as it falls back on itself.

Step 6

Pour in chocolate mixture. Gently fold two mixtures together with a rubber spatula by scooping underneath, then cutting through the middle with the side of the spatula. Continue to work in this motion juuuuust until batter is well combined and homogenous—take care not to overmix!

Step 7

Pour batter into prepared pan. Bake cake until a thin, dry crust has formed on the surface, and cake has risen and is firmed up but still wobbles ever-so-slightly in the center underneath the crust, 30–40 minutes. Transfer pan to a wire rack and let cake cool in pan about 10 minutes.

Step 8

While cake cools, roast ½ cup raw hazelnuts on a small rimmed baking sheet until lightly browned, 10–12 minutes. Transfer to a cutting board. Let cool, then coarsely chop.

Step 9

Place a flat cutting board or large plate over pan. Using a towel to protect your hands, swiftly invert plate and pan; cake should release on its own. Lift pan off, then peel off parchment.

Step 10

Gently place a flat platter or cake stand onto cake and invert again. Be careful because this cake is very delicate! It’s okay if there are some cracks along the top; the beauty is in the imperfection. Let cake cool to room temperature.

Step 11

Vigorously whisk 1 cup cold heavy cream in a medium bowl until medium peaks form.

Step 12

Sift powdered sugar over cooled cake.

Step 13

Serve cake with whipped cream and hazelnuts alongside.

Step 14

Do Ahead: Cake (without powdered sugar) can be made 1 day ahead. Cover with plastic and store at room temperature.