Prep: Adjust the oven rack to the bottom-middle position before preheating. Preheat the oven to 170℃ / 340℉. Line the base ONLY of an 8" round (anodised aluminium) bake pan with parchment paper. Do not grease.
Separate Eggs: Separate 3 large eggs into 2 large bowls. Whisk the yolks and set aside.
Beat Egg Whites: Whisk egg whites on medium speed until frothy, then gradually add sugar until fully incorporated.
Whip To Soft Peaks: Beat egg whites into meringue of soft peaks. Where the peaks gently bend or fall over when the whisk is lifted. Do not over whip. No need to wash the whisk, use for the next step.
Yolk Mixture: In the bowl with yolks, add milk, vegetable oil and vanilla. Whisk until just combined. Sift in cake flour and baking powder and add a pinch of salt. Whisk or fold with a spatula until just combined.
Combine whipped egg whites (meringue) and egg yolk mixture: Add a third of the meringue to the egg yolk mixture and whisk/fold until just combined. Pour this lightened batter into the remaining meringue and gently fold until no white streaks remain.
Fill Bake Pan: Pour the batter into the prepared cake pan and tap the pan on the bench to remove any large air bubbles.
Water Bath: Place the cake pan in a larger oven-safe pan and pour hot water into the outer pan until it reaches about 1 inch up the sides of the cake pan. It's fine if the cake pan floats.
Bake: Bake at 170℃ / 340℉ on top and bottom heating (i.e. without fan/conventional) for 20-22 minutes or until the top is lightly golden brown. Check for doneness by inserting a skewer into the center of the cake; it should come out clean. Remove from oven.
Release Cake: Allow the cake to cool completely inside the bake pan or inverted on a wire rack. Either method is fine. The cake will pull away from the side walls as it cools.
Invert and Prepare Cake: Carefully invert the cooled cake onto a cake stand or serving plate. Wrap an acetate collar snugly around the cake and secure with tape to contain the mousse and gel. Set aside.