**Note: brown the butter for the chocolate chip cookie layer first and refrigerate it for 60 minutes before you begin mixing the cookie dough and brownie batter.**

For Chocolate Chip Cookie Dough:
To brown the butter, place butter in a medium saucepan over medium heat. Stir occasionally until butter has melted. Continue stirring often as butter begins to foam and crackle. Do not leave the butter unattended at this stage. Stir the butter as it sizzles and after about 4-5 minutes, bits of golden brown butter will fall to the bottom of the pan. The crackling sound will stop and the butter will be aromatic and amber in color.
Remove the butter from the heat and pour it into a heat-safe bowl. Place the bowl of brown butter in the refrigerator for about 60 minutes or until cooled and firm to the touch. You can speed this up after about 30 minutes by placing the bowl in the freezer.
Once cooled, spoon the brown butter into the bowl of a stand mixer fitted with the paddle attachment.
Add the sugars and beat on medium-high speed for 2 to 3 minutes until light and fluffy.
Add the egg and vanilla and mix until well combined.
In a separate bowl, whisk together the flour, baking soda, and salt.
With the mixer on low speed, gradually add the flour mixture in 3 additions. Scrape down the sides of the bowl as needed. Do not over mix. A few streaks of flour remaining is ideal.
Remove the bowl from the stand mixer and use a spatula to fold in the chocolate chips. Set aside until ready to assemble.

For Brownie Layer:
Preheat oven to 350°F.
Spray an 8 x 8 inch pan with non-stick baking spray and line the bottom and sides with parchment paper. Allow the edges of the parchment to extend over the sides of the pan for easy removal of the bars once cooled.
In a medium saucepan over low heat, melt the chocolate and butter together. Stir often until the butter is melted and the chocolate is smooth and shiny. Remove the pan from the heat and set aside to cool slightly.
In the bowl of a stand mixer fitted with the whisk attachment, beat together the egg, egg yolk, sugars, and vanilla on high speed for 4 to 5 minutes until the mixture is thick and pale in color.
Reduce the speed to low and add in the cooled, melted chocolate and butter mixture. Mix until incorporated.
In a separate bowl, whisk together the flour, cocoa, baking powder, and salt.
Remove the bowl from the mixer and use a spatula or wooden spoon to fold in the dry ingredients until just barely combined. Do not over mix.

To assemble the brookies, start by spooning about 1/3 of the brownie mixture into the bottom of the prepared pan using an offset spatula to roughly smooth it evenly across the bottom of the pan.
Alternate spoonfuls of chocolate chip cookie dough and brownie batter across the pan, creating a marbled look. Continue until all of the cookie dough and brownie batter are used.
Gently press the mixture down with an offset spatula, leveling the top just slightly. Do not overwork the dough or flatten the brookies too much.
Bake at 350°F for about 25 minutes or until the edges are golden brown and the center is set. If the cookie dough begins to brown too quickly, place a piece of aluminum foil loosely across the top and continue baking.
For firmer brookies, bake for closer to 30 to 35 minutes. For gooey brookies, bake for less time.
Remove the pan from the oven and cool completely before slicing.
For sharp edges and even slices, chill the brookies for one hour before slicing with a sharp knife.

Notes: Store tightly covered at room temperature for up to 4 days. Brookies freeze well. Wrap tightly in a layer of plastic wrap followed by a layer of aluminum foil. Defrost at room temperature.