Preheat your oven to 350°F (175°C) and grease a cake pan.

In a medium bowl, mix together the toasted and chopped hazelnuts, granulated sugar, and cocoa powder. Add 1/2 cup of heavy cream to the mixture and stir until combined.

In another bowl, beat the egg whites until they form stiff peaks.

In a separate bowl, beat the egg yolks with vanilla extract until the mixture thickens.

Melt the dark chocolate and butter together in a microwave-safe bowl or double boiler; then fold it into the egg yolk mixture.

Gently fold the hazelnut mixture into the batter, followed by the beaten egg whites. Be careful not to deflate the mixture to maintain fluffiness.

Pour the batter into the prepared pan and bake for 25-30 minutes. Once done, let it cool completely before removing from the pan.

To prepare the glaze, heat 1/2 cup of heavy cream and mix it with powdered sugar and dark chocolate until you achieve a smooth consistency. Pour the glaze over the cooled cake.

Chill in the refrigerator before serving, and enjoy every bite of this heavenly creation!