Sieve maida with baking powder& baking soda twice so that soda is evenly distributed.
In a mixing bowl Cream the butter and sugar till light and fluffy.
Add milk and condensed milk mix well until creamy.
Pour hot water on cocoa powder and mix well to form a paste,let it cool.
Add sifted maida flour to the sugar butter mix . Mix gently such that everything blendds well .
Now divde the cake dough to 2 parts . Add 1/4th portion of the cake batter to the chocolate mix and add vanilla essence to the remaining 3/4th cake mix . Now both vanilla n chocolate cake batter is ready .
Preheat the oven at 170 c for 10 mins. Line the baking tray with butter paper and grease with oil and keep aside.
Pour all of the white batter into the prepared pan.Top with the chocolate batter in circles as shown below. Then with a fork/knife make marble like patterns - dont overdo.
Now bake at 170 deg c for 40 minutes or until a toothpick inserted comes out clean.Set aside to cool down completely on a wirerack and then cut into slices.
Notes
For a healthier version replace maida with wheat flour. But it would be little dense.
The cake will be very soft . So make sure you cool it well before cutting .. else they will be crumbly .
if you want you can blend butter , regular sugar and condensed milk in a mixer . That way ity will get well creamy