Preheat your oven to 350°F (175°C).
Line your 9x13 inch baking pan with parchment paper, leaving some overhang.
In a medium saucepan over low heat, melt 1 cup of unsalted butter and 4 oz of bittersweet chocolate together. Stir until smooth and remove from heat to cool slightly.
In a large mixing bowl, combine the melted chocolate mixture, 1 cup granulated sugar, 1 cup packed light brown sugar, 1 teaspoon vanilla extract, and 1 teaspoon salt. Mix until well combined.
Add in the 2 large eggs one at a time, mixing thoroughly after each addition. Then, gently fold in 1 cup of all-purpose flour until just combined. Set aside.
In a separate bowl, cream together 1 cup (1 stick) softened unsalted butter, 1 cup granulated sugar, 1 cup packed light brown sugar, 1 teaspoon vanilla extract, and 1 teaspoon salt until light and fluffy.
Beat in 1 large egg until fully incorporated. Then, add 1 teaspoon baking powder, 1 teaspoon baking soda, and 1 cup all-purpose flour. Mix until just combined, then fold in 1 cup semi-sweet chocolate chips.
Pour the brownie batter into the prepared baking pan and spread it evenly. Dollop spoonfuls of the cookie dough over the brownie layer. Use a spatula to gently spread the cookie dough over the brownie batter.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the Brookies to cool in the pan for about 20 minutes, then use the parchment paper to lift them out and cool completely on a wire rack. Once cooled, slice into squares and enjoy!