01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a heatproof bowl, melt butter and dark chocolate together over a pan of simmering water or in the microwave. Stir until smooth. Cool slightly.
03 - Whisk sugar into the chocolate mixture, then add eggs one at a time, followed by vanilla. Fold in flour and salt until just combined.
04 - Spread the brownie batter evenly into the prepared pan.
05 - In a medium bowl, cream softened butter with brown sugar and granulated sugar until light and fluffy.
06 - Beat in egg yolk and vanilla extract until well incorporated.
07 - Add flour, baking soda, and salt; mix until just combined. Fold in chocolate chips.
08 - Drop spoonfuls of cookie dough evenly over the brownie batter. Gently spread or swirl with a spatula for a marbled effect, if desired.
09 - Bake for 32–36 minutes, or until the top is golden and a toothpick inserted in the center comes out with moist crumbs.
10 - Cool completely in the pan on a wire rack before slicing into squares.