Preheat oven to 340 degrees F (170 degrees C).
Sift All-Purpose Flour (75 gram), Baking Powder (1 teaspoon) and Salt (1/8 teaspoon) together in a bowl.
In another bowl, whisk the yolks of Egg (3) and Caster Sugar (65 gram) vigorously for a minute until pale and creamy.
Drizzle in Vanilla Extract (1 1/2 teaspoon) and whisk.
Then stir in Milk (60 milliliter) and Vegetable Oil (60 milliliter).
Lastly, gradually spoon in flour mixture. Set aside.
In your mixer bowl, pour in 3 Egg Whites and Distilled White Vinegar (3/8 teaspoon) together. Whisk on high speed until frothy.
Then gradually spoon in Caster Sugar (65 gram) and continue mixing until stiff peaks.
To combine, fold in egg white mixture a dollop at a time.
Transfer them into your square paper liners. Drop the pan to raise large air bubbles out.
Bake it in the oven for 20 to 25 minutes. Let it cool while you make your filling.
To make the custard, heat up the Milk (60 milliliter) until just below boiling point. In a bowl, whisk the yolks of the remaining Egg (2), Caster Sugar (65 gram), Corn Starch (35 gram) and Fine White Flour (1 tablespoon) vigorously for a minute until pale and creamy.
Slowly add in half of your hot milk while whisking.
Transfer your egg mixture back to the pot. Cook on medium-low heat whisking constantly until thickens.
Shut off the heat and immediately transfer into a bowl. Add in vanilla extract.
Cover it with a cling wrap, touching the custard and let it chill in the fridge.
Once everything is cooled, whip Whipping Cream (120 milliliter) and Caster Sugar (3 tablespoon) on high speed until stiff peaks.
Throw in cooled custard and whisk on medium speed for another 20 to 30 seconds until combined.
Transfer into a piping bag and pipe into the chiffon cakes.
Dust with Powdered Confectioners Sugar (to taste).