Preheat the oven to 180°C (356°F) and line the bottom of the springform pan.

Whip the egg whites until stiff peaks form, gradually adding 75 g granulated sugar.

Fold 48 g wheat flour into the whipped egg whites until no dry flour remains.

Spread 150 g roasted hazelnuts over the pan bottom, then cover them with the egg-white batter.

Bake the hazelnut sponge for 10 minutes, then cool it completely in the pan.

Whisk the egg yolks with 100 g granulated sugar until smooth.

Whisk the vanilla pudding powder, 16 g wheat flour, and a small amount of the milk into the yolk mixture.

Bring the remaining milk and vanilla sugar to a boil in the saucepan.

Whisk the yolk mixture into the boiling milk, then cook over low heat until thick.

Cover the pudding directly with plastic wrap, then cool it completely.

Beat the room-temperature butter with 50 g powdered sugar until pale and fluffy.

Beat the cooled pudding into the butter mixture until the vanilla cream is smooth.

Divide the vanilla cream into two equal portions, then beat 40 g Nutella hazelnut spread into one portion.

Turn the cooled sponge upside down so the roasted hazelnuts face upward, then spread 60 g Nutella hazelnut spread over it.

Spread the plain vanilla cream evenly over the Nutella-coated sponge, then arrange the broken petit beurre biscuits on top.

Spread the Nutella cream over the biscuits, then refrigerate the cake for at least 3 hours or overnight.

Remove the cake ring and decorate with whipped cream if desired.