Preheat oven to 350°F (177°C).
Line a 12×18 inch sheet pan with parchment paper.
Prepare the Cake Batter:
Separate egg yolks and whites.
Beat egg yolks with 1/2 cup sugar until pale yellow and thick.
Add vegetable oil, milk, and vanilla extract; mix well.
Sift cake flour, cocoa powder, baking powder, baking soda, and salt; add in thirds to the egg yolk mixture.
Beat egg whites with cream of tartar until foamy, then gradually add 1/4 cup sugar; beat to stiff peaks.
Fold egg whites into the batter in thirds.
Bake the Cake:
Spread batter evenly in the prepared pan.
Bake for 10-12 minutes. Check doneness with a light touch or a toothpick.
Cool for 5 minutes, then transfer to a powdered sugar-dusted kitchen towel and roll up to cool completely.
Prepare the Cream Filling:
Chill mixing bowl and whisk.
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