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 the melted butter with granulated sugar and brown sugar until well combined. Add the eggs and vanilla extract, whisking vigorously until the mixture is glossy and smooth.
03 - Sift the flour, cocoa powder, and salt directly into the wet ingredients. Stir gently just until combined — do not overmix. Fold in the chocolate chips.
04 - Pour the brownie batter into the prepared pan and spread it into an even layer across the bottom.
05 - In a medium bowl, beat the softened butter with brown sugar and granulated sugar using a hand mixer until light and fluffy, about 2 minutes. Beat in the egg and vanilla extract until fully incorporated.
06 - In a separate small 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.
07 - Drop spoonfuls of the cookie dough evenly over the brownie layer. Gently spread and swirl with a spatula to cover — a few gaps are perfectly fine and create a beautiful marbled look.
08 - 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 attached — not wet batter.
09 - Let the brookies cool completely in the pan before using the parchment overhang to lift them out. Cut into 16 even squares.