With the rack in the middle position, preheat the oven to 180 °C (350 °F). Lines the bottom of a 20-cm (8-inch) square baking dish with a strip of parchment paper, letting paper hang over two sides. Butter the other two sides.
Pour brownie batter into the prepared pan. Set aside.
In a bowl, beat cream cheese, sour cream and icing sugar using an electric mixer until smooth.
With a spoon, spread cheese mixture into 9 spoonfuls, side by side, on top of the cake. Run a knife through batter to create a marbled effect. Tap the pan on a work surface to level out batter.
Bake for about 35 minutes or until a toothpick inserted in the centre of the cake comes out with a few lumps and not completely clean.
Let cool in the pan. Remove from pan and cut into squares.