01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the baked brookies out easily.
02 - In a large bowl, whisk the melted butter with granulated sugar and brown sugar until well combined. Add the eggs and vanilla extract, whisking until the mixture is smooth and glossy. Sift in the flour, cocoa powder, and salt, then fold gently until just combined. Fold in the 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 the softened butter with brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until fully incorporated.
05 - In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the butter mixture, stirring until just combined. Fold in the chocolate chips.
06 - Drop spoonfuls of the 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, or until the top is lightly golden and a toothpick inserted into the center comes out with moist fudgy crumbs, not wet batter.
08 - Let the brookies cool completely in the pan before using the parchment overhang to lift them out. Cut into 16 even squares.