Preheat the oven: Preheat to 170°C. Line a 28×28cm square baking pan with parchment paper for easy removal.

Make the chocolate batter: Heat 50g corn oil to 60°C in a mixing bowl. Add in 8g cocoa powder and mix until smooth. Add 52g milk and stir until combined. Sift in 60g cake flour, and mix until no dry flour remains. Add 5 egg yolks and mix until smooth.

Whip the egg whites: Place 5 egg whites into a clean, dry bowl. Add a few drops of lemon juice and beat with an electric mixer, gradually adding 55g granulated sugar. Beat until soft peaks form (the egg whites should hold their shape but have a slight curl).

Combine batter and egg whites: Gently fold in one-third of the meringue into the chocolate batter using a spatula, making sure to scrape from the bottom and mix quickly to avoid deflating the meringue. Once combined, pour the batter back into the remaining meringue. Continue to fold in the same way, scraping the bottom and gently incorporating the meringue until the mixture is smooth and fully blended, forming a delicate and smooth cake batter.

Bake the cake: Pour the batter into the prepared pan, smooth the surface, and tap lightly to release air bubbles. Bake at 170°C for 25–30 minutes, or until a toothpick inserted in the center comes out clean.

Cool and slice: Let the cake cool slightly, then turn it onto a cutting board and remove the parchment paper. Once fully cooled, slice the cake into four even layers.

Prepare the ganache: Chop 100g dark chocolate into small pieces and place in a heatproof bowl. Heat 110g heavy cream until small bubbles appear at the edges (about 70°C), then pour over the chocolate.

Add rum: Stir in 5g rum for added depth of flavor. Cover and refrigerate until needed.

Prepare the hazelnuts: After the hazelnuts are roasted and completely cooled, place them in a sturdy resealable bag. Lay the bag flat on a stable surface and use a rolling pin to crush the hazelnuts. Then, gently tap them with the rolling pin to break them into smaller pieces.

Make whipped cream: Pour 210g heavy cream into a clean mixing bowl, and add 30g granulated sugar, and the entire batch of chocolate ganache. Using an electric mixer on low speed, mix until the sugar and ganache are fully dissolved into the cream. Next, add 20g chocolate shavings and gently fold them in with a spatula. Transfer the mixture into a piping bag.

Layer the cake: Place one cake layer on a serving plate or turntable. Pipe a layer of chocolate whipped cream evenly on top and sprinkle with crushed hazelnuts. Gently press hazelnuts into the cream to adhere.

Repeat layers: Repeat the process for the remaining layers, then evenly spread the remaining chocolate whipped cream over the entire surface of the cake. Use a spatula to smooth out the chocolate cream, ensuring an even and polished finish. Refrigerate the assembled cake for 20 minutes to set.

Decorate: After the cake has set in the fridge, remove it from the refrigerator. Melt 150g of dark chocolate and spread it over the surface of the cake. Press the remaining crushed hazelnuts onto the chocolate layer, and optionally place 4 small pieces of chocolate blockson top of the cake. Use a knife to cut the cake into appropriate-sized pieces and enjoy!