1.Preheat oven to 170°C (150°C fan-forced).
2.Grease a 20cm x 30cm rectangular pan; line base and sides with baking paper, extending 2cm above sides of pan.
3.In a medium saucepan, add chocolate and butter; stir over low heat for 2-3 minutes, or until smooth. Whisk in cocoa. Transfer to a medium bowl; allow to cool for 10 minutes.
4.Whisk egg, sugars and extract into chocolate mixture, then stir in flour, nuts and both chocolates.
5.Pour mixture into pan. Bake for about 45 minutes. Cool cake in pan before cutting into squares.
6.Dust with icing sugar before serving.
Brownie will keep in an airtight container at room temperature for 4 days; in the refrigerator for 1 week; or frozen for up to 2 months.Note