Preheat the oven to 160ºC.
In a small bowl, mix together egg yolks and vanilla extract.
In another mixer bowl, beat the egg whites until 80% stiff (or soft peaks), adding sugar in 3 separate times.
Transfer the egg yolk mixture to the meringue. Whisk till well combined.
Sift in cake flour. Fold the mixture until smooth with no lumps.
In a separate bowl, add in vegetable oil, milk, and a small amount of the batter. Mix well.
Pour the mixture back to the batter, add in salt, and fold until you get a smooth, frothy, and silky texture.
Pour the batter into a 6-inch cake tin lined with parchment paper.
Use a skewer, toothpick, or baking needle to swirl through the batter and tap the cake tin against the table several times. This step helps release air bubbles trapped in the batter.
Bake in the preheated oven at 160ºC for 40-45 minutes.