Whisk yolks + sugar until pale and smooth.

Add oil, milk, and vanilla; whisk until glossy.

Sift in flour and cornstarch; whisk until thick and well combined.

Set aside at room temp while preparing egg whites.

Use a clean, dry bowl.

Beat egg whites on medium until frothy, add vinegar.

Increase to high speed, slowly add sugar.

Whip until stiff peaks form (5–7 minutes).

Divide whipped egg whites into thirds.

Gently fold one-third at a time into the egg yolk mixture with a rubber spatula.

Be careful not to deflate the batter.

(Optional) Pipe designs on parchment with colored batter, freeze 5–8 minutes.

Add base batter on top.

Bake at 375°F for 8–10 minutes until golden at edges.

Flip onto parchment, peel back to reveal designs.

Layer with whipped cream or fruit; decorate with top layer.