Line two 24–26 cm cake pans with parchment paper and preheat the oven to 180°C.

Place 300 g caramel candies in a saucepan and pour over 300 ml of the whole milk.

Heat the candies over low heat until melted, then stir in 1,000 ml whole milk and bring the mixture to a boil.

Whisk 10 egg yolks with 220 g sugar until pale, then whisk in cornstarch, vanilla pudding powder, and the remaining 200 ml milk.

Whisk the yolk mixture into the boiling caramel milk, then cook while stirring until the cream is very thick.

Transfer the caramel cream to a bowl and press plastic wrap directly onto its surface, then refrigerate until completely cold.

Beat 5 egg whites with half the salt until foamy, then gradually beat in 110 g sugar until stiff glossy peaks form.

Sift 22.5 g flour, 12 g cocoa powder, and 5 g baking powder together, then fold the dry mixture into the egg-white foam.

Fold in 150 g roasted hazelnuts and 80 ml vegetable oil, then spread the batter evenly into one prepared cake pan.

Bake the first sponge at 180°C until set and a tester comes out clean, about 18–20 minutes.

Repeat the sponge process with the remaining egg whites, salt, sugar, flour, cocoa powder, baking powder, hazelnuts, and vegetable oil.

Cool both sponges completely, then split each sponge horizontally with a serrated knife to make four layers.

Beat 200 g softened butter into the cold caramel cream until smooth, then fold in the whipped 300 ml heavy whipping cream.

Place one sponge layer on a serving plate and spread with caramel cream, then repeat with the remaining layers.

Cover the top and sides with the remaining caramel cream, then refrigerate the cake until firm before serving.