Preheat the oven to 375℉.
Line a baking sheet of 15X10X1 inch and grease the parchment paper.
Just like any other sponge cake, start by separating the egg yolks. Beat the yolks until pale and double in size. This might take 4-5 minutes.
4 Yolks
Gradually add ⅓ cup of the granulated sugar and keep whisking. The mixture will become thick and pale in colour.
⅔ cup Granulated sugar
Add the oil and mix.
2 tablespoon Oil
Sift all dry ingredients twice to aerate the flour and gently add them to the yolk mixture. The batter will be thick, and that is okay.
½ cup All-purpose flour, 2 tablespoon Cocoa powder, 1 teaspoon Baking powder, ¼ teaspoon Salt
Wipe the mixing bowl and the whisk attachment with white vinegar, and whip the egg whites until soft peaks form.
4 White
Gradually add the remaining ⅓ cup sugar and mix until the mixture reaches the stiff peaks.
Gently mix the yolk mixture into the egg white mixture and mix. Do not overbeat and overmix the mixture because it will cause deflating the batter. Spread the batter onto the baking sheet and smooth the surface with a spatula.
Bake the sponge for 10-12 minutes, then let it cool down ONLY for Five minutes. Put a clean kitchen towel on the clean counter, and sift about two tablespoons of cocoa powder on the towel.
Flip the baked cake on the towel and peel off the parchment paper. Start rolling the cake in a jelly-roll style from the short side. Let the log cool down completely on the cooling rack!