Prepare the cocoa mixture: In a large mixing bowl, combine 10 g cocoa powder and 35 g corn oil. Whisk thoroughly until the mixture is completely smooth with no lumps. Add 40 g milk and continue whisking until well combined.

Add dry ingredients and egg yolks: Sift 45 g cake flour into the cocoa mixture. Fold gently with a spatula until no dry flour remains. Add 3 egg yolks and mix until you have a smooth, silky yolk batter. Set aside.

Whip the egg whites: In a separate, very clean, grease-free and water-free large bowl, place 3 egg whites. Make sure no yolk has accidentally gotten into the whites. Add 45 g granulated sugar. Using an electric hand mixer, whip the egg whites until they reach stiff peaks. This means when you lift the beater, the meringue forms a peak that stands straight up without drooping. The meringue should be glossy, fine-textured, and not runny.

Fold the meringue into the batter: Transfer about half of the whipped meringue into the yolk batter. Use a spatula to fold gently until just combined. Then, pour this mixture back into the bowl with the remaining meringue. Continue folding gently with a cutting motion until no white streaks remain and the batter is smooth, airy, and uniform in color. Be careful not to overmix, as this will deflate the meringue.

Bake the cake: Pour the batter into a 6-inch chiffon cake mold. Do not grease the mold—chiffon cakes need to cling to the sides to rise properly. Gently tap the mold on the counter 3–4 times to release any large air bubbles. Place in a preheated oven and bake at 150°C for 45 minutes.

Cool and slice: Once baked, remove the cake from the oven immediately. Invert the cake onto a cooling rack (turn it upside down) and let it cool completely. This step is crucial to prevent the cake from collapsing. Once completely cooled to room temperature, carefully unmold the cake. Using a serrated knife, horizontally slice the cake into 3 even layers. Set aside.

Whip the cream: Place 400 g of chilled heavy cream into a clean, grease-free and water-free bowl. Add 15 g granulated sugar. Using an electric hand mixer, whip the cream until it reaches stiff peaks. The cream should have distinct lines, hold its shape firmly, and be suitable for frosting without drooping or melting.

Add Oreo crumbs: Add 40 g of Oreo crumbs (cream-free version) to the whipped cream. Use a silicone spatula to fold gently until the crumbs are evenly distributed throughout the cream. Do not overmix, as this can cause the crumbs to dissolve and thin out the buttercream. Transfer the Oreo salted buttercream into a piping bag and set aside.

Assemble the first layer: Place one cake layer on a turntable or serving plate. Pipe or spread a generous layer of Oreo salted buttercream over the cake layer, using an offset spatula to smooth it evenly. Pipe a ring of buttercream around the edge of the cake layer. Sprinkle Oreo crumbs in the center of the ring as filling. Pipe another thin layer of buttercream over the Oreo crumbs and smooth with the spatula.

Repeat for the second layer: Place the second cake layer on top. Repeat the same process: spread buttercream, pipe a ring around the edge, sprinkle Oreo crumbs in the center, and add another thin layer of buttercream on top.

Frost the top and sides: Place the third and final cake layer on top. Use the remaining Oreo salted buttercream to frost the entire cake—both the top surface and the sides. Use an offset spatula to create smooth, even coverage.

Decorate: Decorate the surface of the cake with whole or half Oreo cookies and fresh blueberries for contrast. Transfer chocolate sauce into a piping bag and snip a tiny opening. Gently drizzle the chocolate sauce along the edges of the cake, allowing it to drip down naturally. For a cleaner look. The Oreo Chocolate Chiffon Cake is now ready to be served.