Preparation of Vanilla Dough:
Sift together flour, sugar and salt and transfer them into the bowl of dough mixer.
Add butter and vanilla to the mixer bowl and blend them together at low speed. When the mixture gets the shape of smooth dough, stop the mixer and transfer the dough to another bowl, cover it and leave for at least 15 mins.
Preparation of Chocolate Dough:
For chocolate dough sift together flour, salt, sugar and cocoa and transfer the mixture to the mixer bowl.
Add butter vanilla to the mixture and mix them together at low speed to prevent from flour dusting. When a soft dough would form similarly transfer it to another bowl, cover it and leave for 15 mins.
Preparation of Cookies:
Divide each dough into three equal pieces and take one piece of chocolate and vanilla dough to make first batch of cookies.
Flatten these dough separately to a rectangular shape of thickness approx. 1/2 inch. Place flatten chocolate dough onto the vanilla dough and then start rolling both of the dough together from one corner to another.
Using knife make 4 pieces of the rolled dough. Take one of the piece and press it to circular shape, flatten it further to make it 1/4 inch thick. Using round cookie cutter give it perfect round shape. Similarly prepare rest of the cookies.
Place these cookies on a greased baking tray and it is better to dust the tray after oiling. Put the tray in preheated oven at 325 degrees F for just 10-12 mins.