Preheat oven to 180 degree C.
Mix flour, baking powder and salt.
Mix eggs with vanilla and set aside.
Take butter and sugar in a bowl and beat till fluffy.
Add eggs and beat till fluffy.
Add in half of the flour mix and milk and fold gently.
Add in remaining flour and fold again.
Now remove half of the batter into a bowl. To one bowl add cocoa and fold gently.
Take a cake pan, line with parchment paper. Spoon some vanilla batter, then chocolate batter. repeat this way till it is completely done.
Now tap the pan in counter few times to remove air bubble.
Now pop it into the oven and bake for 35 to 40 mins.
Remove and cool completely.
Slice and serve