Preheat oven to 350 F (180 C). Line 9X13 inch with parchment paper.
Make sure the melted butter has come to room temperature.
Add brown sugar, white sugar to the melted butter. Mix well.
Whisk in eggs and yolk to butter-sugar mixture. Mix until creamy.
In a bowl mix flour, salt and soda.
Add dry ingredients to the wet mixture. Mix until everything is incorporated.
Reserve 1 cup cookie bough to be mixed later. To the rest of the dough mix 3/4 cup chocolate chips and keep aside.
In a separate bowl add 1/4 cup melted butter and cocoa powder. Mix well.
Add 1 cup reserved cookie dough to the butter cocoa mixture. Add 1/4 cup chocolate chips. Mix well.
Using a spatula, evenly spread the Chocolate Cookie dough at the bottom.
Then spoon rest of the cookie dough on top and carefully spread it to an even layer.
Bake in a preheated oven at 350 F (180 C) for 18- 20 minutes.
At the end of the baking time, the edges will look set but it will still be soft at the middle. But once it cools down the centre will set as well.