In a large mixing bowl, whisk together the egg yolks, cream of tartar, and 40 g caster sugar until smooth and slightly pale. Add the milk, vegetable oil, and vanilla extract, mixing until well combined.

Sift the cake flour, baking powder, and salt directly into the egg yolk mixture. Gently fold them together using a spatula until smooth and no lumps remain.

In a clean, dry bowl, whisk the egg whites with an electric mixer on medium speed until foamy. Gradually add the remaining 80 g caster sugar a little at a time and continue beating until stiff peaks form. The meringue should be glossy and hold its shape firmly.

Carefully fold one-third of the egg whites into the yolk batter to lighten it. Then gently fold in the remaining egg whites, being careful not to deflate the mixture. Use gentle folding motions until just combined.

Pour the batter into an ungreased chiffon cake pan. Smooth the surface with a spatula. Bake in a preheated oven at 170°C (340°F) for about 35 minutes or until a skewer inserted into the center comes out clean.

Once baked, immediately invert the pan onto a cooling rack and let the cake cool completely upside down to prevent it from collapsing. When cool, run a knife around the edges and carefully remove the cake from the pan. Slice and serve.