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