01 - Preheat oven to 350°F. Line a 9x9 inch square baking pan with parchment paper, leaving a slight overhang on two sides for easy removal.
02 - In a medium bowl, whisk the melted butter and granulated sugar until well combined. Add the eggs and vanilla extract, beating vigorously until the mixture turns glossy and smooth. Fold in the cocoa powder, flour, and salt, stirring until just combined. Spread the batter evenly across the bottom of the prepared pan.
03 - In a separate mixing bowl, cream the softened butter with the brown sugar and granulated sugar until light and fluffy, about 2 minutes. Beat in the egg and vanilla extract until fully incorporated. Add the flour, baking soda, and salt, mixing until just combined. Gently fold in the chocolate chips.
04 - Drop spoonfuls of the cookie dough over the brownie batter, spacing them evenly. Gently flatten and spread the cookie dough to mostly cover the brownie layer—a little brownie peeking through is perfectly fine and creates a beautiful marbled effect.
05 - Bake for 30 to 35 minutes, until the edges are golden brown and the center is just set. Avoid overbaking—the brookies should remain dense and fudgy in the middle.
06 - Allow the brookies to cool completely in the pan before using the parchment overhang to lift them out. Cut into 16 even squares and serve.