Preheat the oven to 170°C(350°F)..
In a mixing bowl add in butter and sugar till fluffy and creamy..
Sift in flour and cornstarch and mix well till it gets combined.
Now divide into two portions and add cocoa powder in one portion and mix well till combined.
Wrap it in plastic wrap and set in fridge for 15 mins..
Take it out of the fridge and form balls out of it..
Now take one plain ball and one chocolate ball and combine them together so that they form a marbled ball..(Check picture).
Arrange them in foil lined tray and press them gently.
Bake them for 15 mins...Remove them and let it cool completely..
Store them in air tight container and serve with a cup of milk..