Preheat oven to 350°F (175°C) and prepare a 9-inch round cake pan with butter and parchment paper.

Sift and whisk together hazelnut flour, cornstarch, espresso powder, and salt in a bowl.

In a stand mixer, whip egg yolks and granulated sugar until thick and pale yellow for 5-6 minutes.

Melt butter in the microwave, let cool slightly, then mix into egg yolk mixture with vanilla.

Whip egg whites in a clean bowl, gradually adding remaining sugar until medium peaks form.

Gently fold half the whipped egg whites into the yolk mixture, then half the hazelnut flour; repeat with remaining ingredients.

Pour batter into prepared pan, bake for 40 minutes or until set and golden brown.

Cool the cake on a wire rack for 10 minutes, then invert to remove from the pan.

For ganache, combine chopped chocolates and Nutella in a bowl; heat cream and glucose until steaming and pour over chocolate, whisking until smooth.

Chill ganache for 4-6 hours, then whip until light and fluffy before spreading on the cooled cake.

Decorate cake with ganache and optional candied hazelnuts before serving.