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 turns glossy and smooth.
03 - Sift the flour, cocoa powder, and salt directly into the wet mixture. Stir gently until just combined — do not overmix. Fold in the semisweet 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, cream the softened butter with light 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 bowl, whisk together the flour, baking soda, and salt. Gradually add the dry mixture to the butter mixture, stirring until just combined. Fold in the chocolate chips.
07 - Drop spoonfuls of cookie dough evenly over the brownie batter. Gently spread and swirl the cookie dough to partially cover the brownie layer — some gaps are expected and desirable for a marbled effect.
08 - 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 clinging to it — not wet batter.
09 - Let the brookies cool completely in the pan on a wire rack. Use the parchment overhang to lift the block out, then cut into 16 even squares.