In a large bowl, blend the butter and sugar for 4 to 5 minutes until creamy. Add egg yolk and Vanilla extract and mix until well combined
Add the floor and salt and using a spatula bring the dough together Wrap vanilla dough in a plastic wrap and shape into a rectangle. Refrigerate the dough for 2 hours
In another large bowl, blend the butter and sugar for 4 to 5 minutes until creamy, add egg yolk and mix until well combined. Add flour, cocoa powder and salt mix everything well together. Wrap chocolate dough in plastic wrap. Shape like rectangle. Refrigerate for 2 hours
Place Vanilla dough on parchment paper, dust with flour. Roll the dough into 9*12 inch rectangle. Do the same for chocolate dough. Both should be of same length and breadth
Flip the Chocolate dough on top of vanilla dough. Rollup into a log using the parchment paper cover and refrigerate for 2 hours
Preheat the oven to 180 degree and line the baking tray with parchment paper
Slice the log into 1/4 inch thick cookies, place the cookies on the prepared baking sheet with a gap between them
Bake cookies for 12 to 14 minutes until the edges are golden brown
Allow the cookies to cool for 10 to 15 minutes on the baking sheet,now cool completely on the wire rack. Store in airtight container. Serve with cup of warm milk
