Prepare the Batter

1. Preheat your oven to 325°F (160°C). Set out sturdy cupcake liners in a muffin pan.
2. In a bowl, whisk together egg yolks, half the sugar, vanilla extract, milk, and oil until smooth.
3. Sift in the cake flour. Stir gently until no lumps remain. Set aside.

Whip the Egg Whites

1. In a clean mixing bowl, beat egg whites on medium speed until frothy.
2. Add cream of tartar (if using) and gradually pour in the remaining sugar.
3. Increase speed to high and beat until stiff, glossy peaks form.

Fold and Fill

1. Gently fold the whipped egg whites into the yolk batter in batches. Use a spatula with broad, sweeping motions to avoid deflating the mixture.
2. Divide the batter evenly into the prepared cupcake liners, filling each about two-thirds full.
3. Smooth the tops with the back of a spoon if needed.

Bake and Cool

1. Place the pan in the preheated oven and bake for 15–20 minutes, or until the tops are lightly golden.
2. Check doneness with a toothpick inserted into the center. It should come out with a few moist crumbs.
3. Remove from the oven and let cupcakes cool slightly in the pan, then transfer them to a rack to finish cooling.

Prepare the Whipped Cream Filling

1. In a chilled bowl, beat heavy cream, powdered sugar, and vanilla on medium-high speed until stiff peaks form.
2. Use a piping bag or small spoon to fill each cooled cupcake with cream. Create a small opening at the top or the side for easier filling.
3. Top with additional whipped cream or preferred frosting, if desired.

Serve and Enjoy

1. Serve these Hokkaido Chiffon Cupcakes immediately or store them in the refrigerator until ready to eat. They’re best consumed within two days for maximum freshness.