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. Add eggs and vanilla extract, beating until fully combined.
03 - Sift flour, cocoa powder, and salt into the wet mixture. Stir gently until just combined. Fold in chopped chocolate.
04 - Reserve 1/4 cup of brownie batter. Pour remaining batter into prepared pan and smooth the top.
05 - Beat cream cheese until creamy. Add granulated sugar, egg, and vanilla extract, mixing until smooth.
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 the two batters together for a marbled effect.
08 - Bake for 32-35 minutes until center is just set and a toothpick inserted comes out with a few moist crumbs.
09 - Allow to cool completely in the pan before lifting out and cutting into squares.