Prepare a classic jelly roll pan - a light-colored 15 x 10 inches (25 x 38 cm) rimmed baking sheet. Line it with parchment paper using this tutorial on how to line a Swiss roll tin. Extend the baking paper for at least 1 inch (2.5 cm) beyond the pan's edges for easy cake removal.

To make the cake batter, separate the egg whites from the egg yolks with an egg separator. Chill the egg whites until ready to use. Ensure to bring the egg yolks to room temperature.

Preheat the oven to 355°F (180°C). Using a flour sifter, sift all-purpose flour, cornstarch, and baking powder in a large bowl and put aside.

In a mixing bowl, whisk egg yolks, 1 1/2 tablespoons (20 g) of sugar, and salt a few times with a hand whisk (avoid whisking until the mixture becomes whitish).

Add oil to the egg yolk mixture and mix. Pour milk and whisk until combined. Sift the flour mixture over the bowl and gently mix with a hand whisk.

In a separate clean bowl, make the meringue. Whisk chilled egg whites with an electric hand mixer until foamy. Gradually add the sugar to the beaten egg whites in three additions. Beat the meringue until soft peaks, but avoid stiff peaks.

Mix the egg yolk mixture once, and add one-third of the meringue. Gently mix with a rubber spatula. Repeat with another third of the meringue. Finally, add the mixture to the remaining meringue and carefully fold.

To make a chocolate batter, transfer 2 cups (218 g) of the cake batter to a medium bowl. In a separate small bowl, mix cocoa powder with room-temperature milk.

Add two spoonfuls of the batter to the cocoa powder mixture and mix with a spatula. Pour it back into the light-colored batter and gently mix.

Pour the chocolate batter into the prepared jelly roll pan and spread it into an even layer with a long spatula or a dough scraper. Gently tap the pan on the table to remove air bubbles.

Transfer the light-colored cake batter to a piping bag and cut the end to about 12 mm in diameter. Hold the bag at an angle to prevent any batter from leaking.

Squeeze the batter in a zig-zag pattern over the chocolate batter layer, going from one long side of the pan to the other. Smooth the batter with a dough scraper.

Bring the baking tray to the center of the oven and lower the temperature to 340°F (170°C). Bake for 20-23 minutes until golden brown. Check for doneness with a toothpick: if it comes out dry, the cake is done.

Prepare a long piece of parchment paper and grease it lightly with neutral oil. Remove the pan from the oven and lightly tap it on the table to prevent shrinkage.

Immediately turn it over onto the parchment paper oiled in advance. Peel off the bottom layer of parchment paper.

Using a long serrated knife, cut the edges of the cake on both long sides. Cut both short sides about 1/2 inch (1.5 cm) diagonally, away from the cake. Allow the cake to cool to room temperature.

To assemble the cake roll, spread Nutella with a thin layer onto the cake, including one angled cut. For easy spreading Nutella, warm it up in the microwave for 20-30 seconds. Leave the opposite angled cut empty. 

Use the parchment paper to start rolling the cake from the short end, covered with Nutella. Make the cake's core, then lift the paper and continue rolling. Finish with the cake seam side down.

Tight the roll with a metal baking sheet, a long offset spatula, or a ruler: watch a video on how to tightly wrap a cake roll.

Cover the rolled cake in parchment paper with the roll's edge at the bottom. Wrap the cake ends with parchment as a candy.

Refrigerate for 1-2 hours. Gently remove the paper from the cake. Make a clean cut at both ends of the roll using a sharp, warm knife. Transfer it to a serving tray. Sprinkle with a little powdered sugar if desired.