1. Preheat oven to 180ºC. Line a 20 x 30cm (base measurement) slab pan with non-stick baking paper.

2. Melt the butter in a saucepan over medium heat. Remove from heat. Stir in sugar. Add eggs, 1 at a time, and stir until mixture is thick and glossy.

3. Sift the flour over the egg mixture. Stir until well combined. Stir in vanilla. Divide the mixture between 2 bowls.

4. Add the dark choc bits and sift the cocoa powder into 1 portion. Stir until well combined. Stir the white choc bits into the remaining portion. Spoon the dark and white chocolate mixtures alternately over the base of the prepared pan. Use a flat-bladed knife to create a swirled effect.

5. Bake for 30 minutes or until a skewer inserted into the centre comes out clean. Set aside in the pan to cool completely. Cut into pieces and dust with icing sugar.