Heat the oven to 350℉. Line a 9 inch springform pan with parchment paper by cutting a circle for the bottom and a separate long rectangle for the sides. (Hold it in place with clips until ready to fill.)

In a saucepan, melt chocolate and butter on medium-low, stirring until melted.

In a mixer on medium-high speed, beat the eggs and sugar for 7 minutes or until tripled in volume. 

With a spatula, fold in the melted chocolate followed by the hazelnut flour. 

Pour batter into the prepared pan and bake for 30-35 minutes or until the top is dry and cracked and a toothpick reveals a slightly wet center. Cool the cake on a wire rack for 10 minutes. Remove the sides of the pan and continue to cool. 

Prepare a 9 inch cake pan for the brittle by lining it with aluminum foil.

Make the brittle by toasting the chopped hazelnuts in a 12 inch skillet for a few minutes on medium heat until toasted and fragrant. Sprinkle the hazelnuts in the aluminum-lined cake pan and wipe out the skillet. 

Pour the sugar in the skillet and spread out in an even layer. Cook on medium-high heat for 3-5 minutes or until the sugar is melted and golden amber, swirling sugar in the pan to cook evenly. (Do not stir.) Quickly drizzle the melted sugar over the hazelnuts in the cake pan coating evenly. 

To serve, break the brittle into shards and top the cake slices with fresh whipped cream.