Preheat oven to 180 deg C.Line a 9" pan with parchment paper and set aside.Sift together all purpose flour,baking powder and salt few times.Keep aside until required.Now place the butter into a large mixing bowl.
Add the powdered sugar and whisk for few minutes until light and fluffy.Scrape the sides of the bowl,in between.Next add the vanilla and mix well.
Crack the eggs open and pour the yolks and whites into a bowl.Add one egg at a time into the butter-sugar mixture and whisk well,until combined.Scrape the side of the bowl before adding each egg.
Now sieve in 1/3 rd of the flour mixture and gently fold in until combined.
Scrape the sides of the bowl and mix in the remaining flour mixture in 2 batches.Do not over mix.
Once the flour is well combined add the milk 1 tbsp at a time.Mix gently to form a smooth and creamy batter.
Transfer 1/3 rd of the batter into another bowl.Add the cocoa powder to this and mix well until combined.
Transfer the vanilla cake batter into the prepared cake tin.Add dollops of the chocolate cake batter on top.
Using a skewer or a fork swirl the chocolate batter into the vanilla.Gently tap the cake tin 1 or 2 times to remove any air bubbles.Bake for about 40 to 45 minutes or until a skewer inserted into the cake comes out clean.Cool in the pan for about 10 minutes.Remove the cake from the pan,peel off the parchment paper and cool completely.