Preheat oven to 180c and line a square baking pan with parchment paper.

Sieve and combine dry ingredients (Flour, baking powder and salt). Set aside

In a large mixing bowl, beat butter and 2/4 Sugar until pale and fluffy with a medium speed.

Add vanilla extract and egg yolks, one at a time, and beat well with each addition.

Lower the speed and add half of flour and half of milk, beat well with each addition, until all flour and milk is incorporated. Stop once all the flour is incorporated and you no longer see streaks of flour. Do not over-cream the batter.

In another large mixing bowl, whisk egg whites until foamy, then add 1/5 sugar in 3 batches. Continue whisking until stiff peak.

Fold the egg white into egg yolk batter in 3 batches with a spatula. Be extra gentle so you don't destroy all the air bubbles from the egg whites.

Pour the batter into prepared cake pan, and bake in preheated oven for 35 minutes, or until a cake tester comes out clean.

Remove cake from oven and let it rest in the pan for 10 minutes.

For Marble Cake - Divide the batter into two portions. Stir the cocoa powder into one portion and mix well. Transfer the plain batter into the greased baking pan. Shake it lightly to distribute evenly. Pour the cocoa batter in the middle of baking pan, and lightly swirl the cocoa batter with a butter knife. I usually do the "S" shape with the butter knife a few times.