01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk melted butter, granulated sugar, and brown sugar in a large bowl until smooth. Beat in eggs and vanilla extract until fully combined.
03 - Sift flour, cocoa powder, and salt into the wet mixture. Stir gently until just combined. Fold in chopped dark chocolate.
04 - Reserve 1/4 cup of brownie batter. Spread remaining batter evenly into prepared pan, smoothing the top.
05 - Beat cream cheese until creamy. Mix in granulated sugar, egg, and vanilla extract until smooth and lump-free.
06 - Spread cheesecake mixture evenly over the brownie batter in the pan.
07 - Drop spoonfuls of reserved brownie batter over cheesecake layer. Use a knife or skewer to swirl batters together for marbled pattern.
08 - Bake 32-35 minutes until center is just set and toothpick comes out with moist crumbs.
09 - Cool completely in pan before lifting out and cutting into squares.