1. Preheat the oven to 350°F. Line a 11 x 11 in pan with parchment paper.
2. In a large bowl, whisk milk and vegetable oil together. Add egg yolks, one at a time, and whisk until combined. Sift in cake flour and cocoa powder, then whisk until incorporated.
3. In a separated bowl using an electric mixer with whisk attachment, beat egg whites until frothy. Then gradually add sugar, and continue beating until firm peak forms.
4. Fold ⅓ of the egg white meringue into the chocolate cake batter, mix until no white streaks. Then add the mixture back into the rest of the egg white meringue, and mix until well incorporated.
5. Pour the batter into prepared cake pan. Tape the pan against the counter a few times to get rid of air bubbles. Bake for 18-20 minutes.
6. While the cake is still warm, cover with a clean sheet of parchment paper and place a baking sheet on top and flip it upside down. Gently remove the parchment paper that is attached to the cake. This side will be the exterior of the roll cake, so make sure to peel it slowly

   

7. Place another sheet of parchment paper and baking sheet on top, and flip it over again. Remove the parchment paper. This side will be the interior of the roll cake. Use a serrated knife and trim the right and left edges (about ½-1 cm) off. Then carefully roll the cake with a kitchen towel or parchment paper. Allow to cool to room temperature.
   

Chocolate Whipped Cream

1. In a large bowl, add heavy whipping cream, sugar, and sifted cocoa powder. Use an electric mixer, beat the cream until stiff and spreadable.

Assembly

1. Once cake is cooled to room temperature, carefully unroll the cake. Spread chocolate whipped cream evenly over the cake, leaving ½ in border on all sides.
2. Re-roll the cake and tightly wrap the roll cake with either kitchen towel, parchment paper, or plastic wrap. Transfer to refrigerator and chill for at least 2 hours before serving. This helps stabilize the whipped cream and hold the cake in shape.
3. To serve, trim the ends of the cake to show the beautiful swirl. Cut into individual serving, transfer to a plate, and enjoy!