Preheat oven to 175°C/350°F and line the base only of a 9" springform pan with parchment paper.
Separate three eggs. Then using an electric hand mixer or stand mixer, beat the egg whites on low-med speed until foamy. Gradually sprinkle in caster sugar (50g) and continue whipping, slowly increasing the speed until soft peaks form. Soft peaks look light and glossy; when you lift the beaters, the peaks should gently fold over and not stand straight. There’s no need to wash the beaters - simply set the whipped egg whites aside and move on to the yolks.
Add caster sugar (50g), to the egg yolks and beat on high until thick and pale yellow. Add the milk and vanilla, then whisk or beat on low until fully combined, scraping down the sides of the bowl as needed.
Sift in the flour and baking powder, add salt, and gently mix with a spatula or whisk until just combined and no flour clumps remain.
Gently fold the egg whites into the yolk mixture in 3 batches using a spatula, until no white streaks remain. The first batch can be folded in a bit more vigorously to loosen the mixture, but fold the remaining batches more gently to avoid deflating the batter.
Pour the batter into a 9″ springform pan, lined with parchment paper on the base only. Tap on the bench top a couple of times to break any large air bubbles.
Bake at 175°C/350°F for 15 minutes, or until a skewer inserted into the center comes out clean. Use conventional baking (top and bottom heat), NOT fan-forced.
While the cake is still warm/hot, carefully release it from the springform pan, remove the parchment paper, and flip it upside down back into the pan. Work gently to avoid burns. It's important to do this while the cake is hot, as pouring the cold milk soak over a hot cake helps it absorb better.
Pour the milk soak mixture over the cake and allow to soak. Set aside in the refrigerator.