Step 1

Preheat oven to 180°C. Brush a square 19cm (base measurement) cake pan with melted butter to lightly grease. Line base and 2 opposite sides with non-stick baking paper, allowing it to overhang. Place dark chocolate and chopped butter in a medium saucepan over low heat. Cook, stirring, for 5 minutes or until chocolate melts and mixture combines (see microwave tip). Stir in sugar and cook, stirring, for 3 minutes or until well combined.

Step 2

Remove from heat and set aside for 5 minutes to cool slightly. Stir in eggs and flour. Add white chocolate and walnuts and stir until combined. Spoon into prepared pan and smooth surface with the back of the spoon.

Step 3

Bake in oven for 25 minutes (a skewer inserted into the centre will come out slightly sticky). Remove from oven and set aside for 3 hours to cool completely. Cut into squares to serve.