• In a mixing bowl, sift flour and baking powder and set aside.
• In another bowl. Combine butter, sugar, salt and vanilla then beat in medium speed using a stand mixer or hand mixer until texture is light and fluffy.
• Add egg yolks gradually while still beating the mixture. Scrape the side of the mixing bowl every now and then to mix well. 
• Add 1/3 part of the sifted flour then mix and fold the batter by hand using a rubber spatula. Then add  1/3 part of milk, mix and fold again. Repeat these process until you finish the sifted flour and milk. 
• Pre-heat oven at 160° C. To prevent cracking of the cake, increase humidity with a tray of water under the grill.

Making the meringue

• Whisk egg white until soft peak. Add sugar gradually while whisking until stiff peak or when the meringue's peak will hold form.
• Whisk slowly for 1 minute more. this will reduces the holes in the cake.

Combining the meringue and batter

• Add 1/3 part of the meringue to the batter. Using wire whisk, mix gently. Pour in the remaining meringue in two batches and fold gently until well incorporated.
• Scoop out about 250 - 300 grams of the cake batter and transfer in another bowl and sift the 10 grams of cocoa into the that batter and mix well.

Baking the Marble Cake

• Line with parchment paper a square baking ban (I used 12 x 12 inches) and pour the white batter and chocolate batter alternately. 
• When you poured all batter, flatten the top with a rubber spatula. Then using spoon or butter knife. swirl the batter to create the marble effect.
• Tap the baking pan on a flat surface at least 2 time to let air bubbles escape.
• Put in the oven and bake for 30 minutes at 160° Celcius, increase temperature to 180° Celcius and let it bake for 15 minutes more. Use toothpick test to check if the cake is done. Cooking time may vary depending on the oven you are using.
• Let the cake cool down before removing from the baking pan. Cut into desired size and serve! Enjoy!