01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a large bowl, whisk melted butter with granulated and brown sugars until well combined. Add eggs and vanilla extract, whisking until the mixture is smooth and glossy. Sift in flour, cocoa powder, and salt, then mix just until incorporated. Fold in chocolate chips.
03 - Pour the brownie batter into the prepared pan and spread it into an even layer across the bottom.
04 - In a medium bowl, cream softened butter with brown and granulated sugars until light and fluffy. Beat in the egg and vanilla extract until fully combined.
05 - In a separate bowl, whisk together flour, baking soda, and salt. Gradually add the dry ingredients to the butter mixture, stirring until just combined. Fold in chocolate chips.
06 - Drop spoonfuls of cookie dough evenly over the brownie layer. Gently spread and swirl to cover the surface, leaving some small gaps for a marbled effect.
07 - Bake for 28 to 32 minutes, until the top is lightly golden and a toothpick inserted into the center comes out with moist fudgy crumbs but not wet batter.
08 - Allow the brookies to cool completely in the pan before using the parchment overhang to lift them out. Cut into 16 squares.