• Preheat oven to 350°F (175°C), using convection heat if you have it. Butter a 9x13-inch (25x40 cm) brownie pan, line bottom and sides with parchment paper and grease the parchment paper with butter.

Make the cookie dough:

• Cream butter and both types of sugar in a large mixing bowl using the paddle attachment of your stand mixer if you have one (or the beaters of your electric mixer).
• Add egg, egg yolk and vanilla and mix to combine.
• In a separate bowl, stir together flour, baking soda and salt. Add flour mixture to the butter mixture and mix on low speed just until flour in incorporated. Stir in chocolate chips.
• Set aside.

Make the brownie batter:

• Melt the chocolate and butter together either in the top of a double boiler or in the microwave (600 watts for 30-second intervals, stirring in between), stirring until both are melted and smooth. Cool until barely warm.
• In a large mixing bowl, beat eggs and sugar together on high speed for 1-2 minutes until pale and thick. Mix in vanilla.
• Add cooled chocolate-butter mixture to egg mixture and mix until combined. Add flour and salt and mix just until combined.

Assembly:

• Spread about ⅓ of the brownie batter evenly in the bottom of the prepared baking pan. Place tablespoonfuls of cookie dough across the top of the brownie batter and fill the spaces in between the cookie dough with the remaining brownie batter. Once you’re done, if there is some dough sticking up anywhere, gently push it down so you have an even thickness of dough and batter throughout the pan. Sprinkle top with a handful of additional chocolate chips if desired.

Bake:

• Cover the pan loosely with aluminum foil and bake at 350°F (175°C) for 20 minutes. Remove the foil and continue baking for an additional 20 minutes, for a total baking time of 40 minutes.

Cool and cut into bars:

• Cool completely in the pan on a wire rack. Use parchment paper to lift the brookies out of the pan. Cut brookies into squares.
• Store tightly covered. Brookies can be stored at room temperature for up to four days or for up to one week in the refrigerator if you wish (for a denser, moister texture) or can also be frozen in freezer bags for up to two months.