• Preheat the oven to 350 F (177 C). Line an 9 x 13 pan with parchment paper along the bottom and two long sides.

  Take another sheet of parchment paper (larger than the pan), press the sheet into the pan. Press the parchment paper gently along the bottom and sides of the pan to create creases in the parchment paper that measures 9 x 13. Remove the parchment paper and set aside for now. 

Chocolate Chip Cookie Dough Layer

• In a mixing bowl whisk together the flour, cornstarch, baking soda and salt until well combined. Set aside for now.
  2 cups (240 g) all purpose flour, 2 teaspoons cornstarch, ½ teaspoon baking soda, ½ teaspoon salt
• Slice the cold butter into tablespoon sized slices. Heat the cold butter in the microwave for 30 seconds. The butter should still be somewhat solid, but will have softened sides. Add the butter to a large mixing bowl fitted with a paddle attachment. Cream together the butter with light brown and granulated sugars on medium speed for 2 - 3 minutes or until well combined. 
  ¾ cup (170 g) unsalted butter, cubed, 1 cup (200 g) light brown sugar, ¼ cup (50 g) granulated sugar
• Mix in the egg and vanilla extract on medium speed for 1 minute until smooth and well combined. Mix the dry ingredients into the wet until just combined. I like to do this about ½ of the dry ingredients at a time. Fold in 1 cup (170 g) of the chocolate chips Reserve about ½ cup (85 g) for topping the brookies right before baking. 
  1 large egg, room temperature, 1 tablespoon (15 ml) vanilla extract , 1½ cups (255 g) chocolate chips, divided
• Press the cookie dough onto a large sheet of parchment paper into a roughly 9 x 13 size rectangle. Transfer the sheet of cookie dough into the fridge to chill while you make the brownie batter.

Brownie Layer

• Make the brownie layer. In a microwave safe mixing bowl add the butter and 1 cup of chocolate chips. Heat in 30 second increments, stirring after each until the butter and chocolate are fully melted and mixed together. Whisk in the espresso powder and set aside for a moment.
  1 cup (226 g) unsalted butter, sliced, 1 cup (170 g) chocolate chips, 2 teaspoons espresso powder
• In another mixing bowl whisk the eggs, sugar, light brown sugar, vanilla extract and salt until smooth and well combined. Whisk in the melted chocolate and butter mixture until smooth and well combined. Whisk in the cocoa powder and flour until smooth and glossy.
  3 large eggs, room temperature, 1 cup (200 g) granulated sugar, ½ cup (100 g) light brown sugar, 2 teaspoons (10 ml) vanilla extract , ½ teaspoon salt, ½ cup (40 g) Dutch processed cocoa powder, ¾ cup (90 g) all purpose flour

Assemble the Brookies

• Evenly pour the brownie batter into the prepared pan. Remove the cookie dough from the fridge. Gently pull the parchment paper away from the cookie dough. Place the cookie dough rectangle on top of the brownie batter. Top the cookie dough with the remaining ½ cup chocolate chips. 
• Bake the chocolate chip cookie brownies for 33 - 38 minutes. Signs the brookies are finished baking are puffed up edges that have slightly pulled away from the edge of the pan and are lightly golden. 
• Remove the brownie chocolate chip cookie bars to cool in the pan at room temperature for 1 hour. Cover and transfer the brookies to the fridge to chill for 1 hour. When it’s time to serve, use the parchment overhang to remove the brookies from the pan, slice and enjoy.