Preheat oven at 360 degrees F (180 degrees C).
Line 10x10 inch baking tray with parchment paper.
Separate the Egg (4) yolks and whites and set aside.
In a large bowl, mix egg yolks, Vanilla Extract (1 teaspoon), Caster Sugar (70 gram), Corn Oil (30 gram), and Milk (30 gram) together with a hand whisk, mix well and add in Cake Flour (70 gram), mix well again. Divide egg yolk batter into two portions and blend the Valrhona® Cocoa Powder (1 tablespoon) with Water (1 tablespoon) mixture into one of the egg yolk batters.
For the meringue, whisk the egg whites until to foamy and add Caster Sugar (50 gram) in 3-4 batches. Continue to whisk egg whites until glossy and peaks form.
Gently fold half of the meringue to the egg yolk batter and cocoa batter with a rubber spatula.
Pour the plain batter on lined pan. Then gently pour cocoa batter on top of the plain batter and spread evenly with a spatula.
Draw lines closely for the entire pan top and down to and from with a ladle handle or use your index finger, then repeat the lines vertically.
Bake in preheated oven 360 degrees F (180 degrees C) for about 15 to 17 minutes.
Remove baking baking paper immediately after baked and leave cake to cool on wire rack with a clean parchment paper below, chocolate side at the bottom.
Trim off the sides of the cake and spread filling, then roll up the cake and chill in the refrigerator for a few hours before slicing. Enjoy!