01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Whisk melted butter, granulated sugar, and brown sugar 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 brownie batter. Pour remaining batter into prepared pan, spreading evenly.
05 - Beat cream cheese until smooth. Add granulated sugar, egg, and vanilla extract, mixing until creamy and well incorporated.
06 - Spread cheesecake mixture evenly over the brownie batter in the pan.
07 - Drop spoonfuls of reserved brownie batter over cheesecake layer. Use knife or skewer to swirl together for marbled effect.
08 - Bake 32-35 minutes until center is just set and toothpick inserted comes out with moist crumbs.
09 - Cool completely in pan before lifting out and cutting into squares.