Take butter, sugar, vanilla in a bowl and cream it well.
Add in flour, salt and mix well. Add in milk and knead it to a soft dough.
Divide it into equal portions. Add cocoa in one dough and knead well.
Now cover the dough with plastic wrap and chill in fridge untill firm.
Take the dough out and cut each one into two portions.
Take one portion and roll it into a rectangle, take each dough and roll it like the same size of the first one.
Now take a vanilla rectangle, brush with little water, place chocolate rectangle on top and brush again with water, place vanilla rectangle over it,
brush with some more water and finally place the chocolate rectangle over it.
Cover the whole thing with plastic wrap and chill untill firm.
Now remove it from fridge, trim the sides, Cut into four equal portions.
Take one strip and brush with water, place another strip, make sure the alternate colours are stacked. Repeat this.
Now wrap and chill for 30 mins.
Now cut into squares. Arrange it in baking sheet which is lined with parchment.
Bake this for 15 to 17 mins in a preheated oven at 170 degree C.
Remove it and let it cool.
Store it in a air tight container. Serve with milk.