1.Preheat oven to 140°C (120°C fan-forced). Grease deep 23cm-square cake pan. Line base and sides with baking paper.
2.Blend or process nuts and chocolate until chopped finely.
3.Beat butter, extract and sugar in small bowl with electric mixer until light and fluffy. Add yolks, one at a time, beating until just combined between additions. Transfer mixture to large bowl; stir in chocolate mixture and rind.
4.Beat egg whites in small bowl with electric mixer until soft peaks form; fold into chocolate mixture, in two batches. Pour mixture into prepared pan; bake, uncovered, about 1¼ hours. Cool to room temperature in pan, cover; refrigerate 3 hours or overnight.
5.Cut cake into squares; serve squares dusted with sifted icing sugar, if desired.