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  1. Preheat the oven to 350°F. Grease a 9" round pan, line it with parchment, and grease the parchment.

  2. To make the cake: Place the egg yolks and sugar in a mixing bowl and beat with a whisk until pale yellow in color and the mixture falls off the whisk in a ribbon. Beat in the vanilla and orange zest. Scrape down the bowl.

  3. Add 1 cup (89g) of the hazelnut flour and the salt. Fold them into the egg mixture by hand, just until combined.

  4. In another bowl with a clean whisk, beat the egg whites until firm peaks form. Using a spatula, fold a generous dollop of the whites into the hazelnut batter to loosen the mixture. Add the remaining egg whites a third at a time, folding them in while you rotate the bowl. After the whites are incorporated, gently fold in the remaining hazelnut flour.

  5. Transfer the batter to the prepared pan and lightly smooth the top. Bake for 28 to 32 minutes, or until a toothpick or cake tester inserted in the center comes out clean and the top of the cake springs back when lightly pressed.

  6. Remove the cake from the oven and let it cool on a rack for 10 minutes. Run a kitchen knife around the edge of the cake to release it from the pan. Tip the cake out of the pan onto the rack and peel the parchment off the bottom. Let it cool completely, bottom side up.

  7. To make the topping: Combine the Nutella and heavy cream in a medium-sized bowl. Beat until smooth, and the cream holds soft peaks.

  8. To serve: Transfer the cake to a serving plate, bottom side up. Finish with a dollop of the topping and a sprinkle of chopped toasted hazelnuts.