01 - Preheat the oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add eggs and vanilla extract, beating until fully combined.
03 - Sift in the flour, cocoa powder, and salt. Stir gently until just combined. Fold in the chopped chocolate.
04 - Reserve about 1/4 cup of the brownie batter. Pour the remaining batter into the prepared pan and smooth the top.
05 - In a separate bowl, beat the cream cheese until creamy. Add granulated sugar, egg, and vanilla extract, and mix until smooth.
06 - Spread the cheesecake mixture evenly over the brownie batter in the pan.
07 - Drop spoonfuls of the reserved brownie batter over the cheesecake layer. Use a knife or skewer to swirl the two batters together for a marbled effect.
08 - Bake for 32–35 minutes, or until the 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.