Preheat your oven to 350 degrees F. Prepare a 9-inch round cake pan by buttering and lining the bottom with parchment paper, ensuring that the parchment is lightly buttered as well. This will help the cake release easily after baking.
In a mixing bowl, sift the hazelnut flour. If there are large bits left over, pulse them in a food processor to create a finer texture, then sift again. You should end up with a total of 300g. Next, whisk in the cornstarch, espresso powder, and salt until well combined, then set aside.
Using a stand mixer fitted with the whisk attachment, whip the egg yolks with 1 cup of granulated sugar on medium-high speed for 5 to 6 minutes until the mixture becomes thick and pale yellow.
In a microwave-safe container, melt the butter. Microwave it at 50% power for 10 seconds, stirring in between until it’s completely melted but not separated. It should be a light yellow and pourable. Let it cool slightly.
With the mixer running on low speed, gradually pour the melted butter into the egg yolk mixture in three to four additions, allowing it to incorporate fully before adding more. Add the vanilla extract and mix until combined.
If you're using a single bowl for your stand mixer, transfer the mixture to a large bowl and clean the mixer bowl thoroughly. If you have two bowls, swap them out and wash the whisk attachment. This step is crucial to achieving fluffy egg whites.
In a clean mixing bowl, whip the egg whites on medium speed until frothy. Gradually add in the remaining 1/3 cup sugar in three or four additions while mixing, then increase the speed to medium-high until medium peaks form—this could take 5 to 10 minutes.
Gently fold a little less than half of the whipped egg whites into the egg yolk mixture. This lightens the mixture. Once incorporated, fold in half of the sifted hazelnut flour until it is almost blended, followed by the remaining egg whites and the remaining hazelnut flour. Be careful not to overmix, which would deflate the batter.
Pour the batter into the prepared cake pan. If your pan has a removable bottom, place it on a baking sheet to catch any drips.
Bake for about 40 minutes. The cake should have set edges, be light golden brown on top, and a toothpick inserted in the center should come out clean.
Once baked, remove the cake from the oven and let it cool on a wire rack. Run a knife around the edge to loosen it before gently removing the sides of the pan, or invert it to release. Allow it to cool completely. You can make the cake ahead of time and refrigerate it overnight or freeze it for up to a month.
For the ganache, place the chopped chocolate and Nutella in a heat-proof bowl. In a small saucepan, warm the cream and glucose over low heat until it just begins to steam—do not let it boil.
Immediately pour the hot cream over the chopped chocolate. Let it sit for 30 seconds before whisking until the chocolate is fully melted and the mixture is smooth. If preferred, an immersion blender works wonders for achieving a silky texture.
Transfer the ganache to a large mixing bowl. Cover the surface with plastic wrap and refrigerate for 4 to 6 hours or overnight until it cools completely.
When the ganache is fully chilled, whip it with an electric mixer for a few minutes until it lightens in color and increases in volume. Be cautious not to overwhip, or it may become too thick. Spread or pipe the ganache onto the cooled cake, and if desired, top with candied hazelnuts for decoration.
Slice and serve immediately, or refrigerate until ready to enjoy. To store leftovers, cover the cut surfaces with plastic wrap or store in an airtight container. The cake keeps well in the refrigerator for up to five days, but I recommend adding the candied hazelnuts just before serving to maintain their crunch.
