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 glossy and smooth.
03 - Sift flour, cocoa powder, and salt directly into the wet mixture. Fold gently until just combined — do not overmix. Fold in the semisweet chocolate chips.
04 - Pour the brownie batter into the prepared pan and spread into an even layer using a spatula.
05 - In a medium bowl, beat softened butter with brown and granulated sugars using a whisk or hand mixer until light and fluffy, about 2 minutes. Beat in the egg and vanilla until fully incorporated.
06 - 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 the chocolate chips.
07 - Drop spoonfuls of cookie dough evenly over the brownie layer. Gently spread with your fingers or a spatula, allowing some brownie batter to peek through for a swirled 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 — not wet batter. Avoid overbaking to maintain the fudgy texture.
09 - Let the brookies cool completely in the pan on a wire rack. Use the parchment overhang to lift the slab out, then cut into 16 squares.