Separate 4 eggs into yolks and whites. Place the egg yolks in a large bowl, set aside on countertop. Place the egg whites in another large mixing bowl, keep in the fridge.
Add 50g cake flour and 40g vegetable oil into a small mixing bowl, mix with a whisk until smooth.
Pour the cake flour vegetable oil mixture (you just made in last step), 40g milk (I used milk tea), and ⅔ tsp of vanilla extract to the egg yolks, mix with a whisk until smooth. Set aside.
Line a square/rectangle baking pan with teflon sheet (or parchment paper). Preheat oven to 340°F (170°C). Take the egg white mixing bowl out from the fridge, add ⅔ tsp of lemon juice (or white vinegar) and 20g sugar into the egg whites. Use medium-low speed (speed 2 out of 6) of a hand mixer to whip the egg whites for 2 minutes until fish eye size bubbles form.
Add another 20g sugar into the egg whites, use medium-low speed to whip the egg whites for 2 more minutes until the bubbles are too tiny to be seen.
Add the remaining 20g sugar into the egg whites, use medium-low speed to whip the egg whites until medium peak forms (*see the pictures in content above for the differences between "soft peaks" and "medium peaks".)
Scoop ⅓ of the egg whites into the egg yolk mixture, fold until well combined.
Transfer the egg yolk mixture to the egg white bowl, gently fold until the color looks even.
Pour the batter into the baking pan, gently spread the batter with a spatula to create a smooth surface.
Bake at 345°F (175°C) for 20 minutes.
Remove the cake pan from the oven, transfer the sheet cake to counter by lifting the four corners of the teflon sheet (or parchment paper). Roll up the hot caken inside the teflon sheet (or parchment paper). Allow the cake roll to cool completely on a wire cooling rack. Use medium high speed of your hand mixers to whip 200g heavy whipping cream with 15g powdered sugar and 1 tsp vanilla extract until stiff peaks form.
Unroll the cooled cake, peel off the teflon sheet (or parchment paper). Spread the whipped cream over the cake, leave about ½ inch border around the cake. Roll the cake back up without parchment paper. Keep in the freezer for at least 2 hours before slicing. Optionally decorate the top of the roll cake with whipped cream, chocolate ganache, fruits, or chestnut cream (check out my chestnut cream cake recipe).