Step 1

Grease the bottom and sides of a springform pan (10-inch diameter) with a pat of butter. Line it with a sheet of parchment paper and butter it as well. Dust it with flour; collect the excess and keep it to the side. In a bain-marie over low heat, melt the butter and 14 ounces of chocolate broken into pieces, stirring continuously.

Step 2

As soon as it is melted, transfer the bowl to a cold work surface and let it cool. Blend the 5 egg yolks with the granulated sugar to obtain a fluid cream; add in the excess flour, combine, and then add in the melted chocolate.

Step 3

Beat the egg whites to stiff peaks and gently fold them into the batter. Pour the batter into the pan and bake at 320°F for 50 minutes. Remove the cake from the oven and let it cool for at least 30 minutes. Remove the cake from the pan, level the top if necessary, turn it over, and let it cool on a grate.

Step 4

In a bain-marie, melt 3.5 ounces of the chocolate with 3 tablespoons of powdered sugar, sifted directly into the bowl, until you obtain a glaze. Pour the glaze in the center of the cake and spread it across the entire surface with a spatula. Chop up the hazelnuts and the remaining chocolate, mix them together in a bowl, and sprinkle them on top of the cake.