1. Line the base of cake pan with paper and lightly grease the sides. Mix cocoa powder and hot water, set aside. Place the butter, sugar and vanilla in the bowl of an electric mixer and beat until light and creamy.
2. Add the egg yolks, one at a time, beating well after each addition. Add the flour and baking powder and beat well to combine. Fold through the milk and combine well.
3. Beat egg whites in a clean large bowl until soft foam. Gradually add in sugar, and beat till soft peak.
4. Take one portion of the egg white mixture, and use a hand whisk to stir well with egg yolk mixture till combine.
5. Slowly fold in the balance egg white with a spatula till well combine.
6. Divide cake batter into two portions ( I divided plain cake batter 500g and 400g for chocolate cake batter) . Add in cocoa mixture into 400g cake batter and lightly mix till combine. Add 1 1/2tbsp milk into plain batter, mix well.
7. Using a small soup ladle, scoop 3 of plain batter into center of cake pan. Then, scoop 3 of chocolate batter onto the center of the plain batter. Use the back of the ladle, smooth out the batter a bit. Continue to scoop another 3 plain and chocolate batter, then reduce to 2 scoops and continue layering and reduce till 1 scoop towards at the end. The circles become smaller and smaller as you working with the batter. Lightly tap on worktop to remove air bubbles.
8. Bake in a preheated oven at 180c for 50mins or until the skewer insert comes out clean.
9. Let cake cool on a wire rack before remove, don't need to wait completely cool.
Whip eggs, castor sugar and salt with wire whisk at medium speed until light and fluffy. Fold in sifted flour mixture (all purpose flour and baking powder) with rubber spatula until well combined. Add oil lastly and mix until well combined. Pour into greased and floured cupcake moulds, or paper lined muffin tin. Bake 150C for 25 minutes. Remove cakes from the moulds immediately after baking. Set aside to cool.