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Preheat oven to 180°C. Grease a 20cm x 30cm lamington pan; line base and sides with baking paper, extending paper 5cm above long sides.
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Stir chocolate and butter in a medium heatproof bowl over a medium saucepan of simmering water (do not allow the water to touch base of bowl) until melted. Allow to cool.
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Combine chocolate mixture and remaining ingredients in medium bowl. Beat on low speed with electric mixer until combined. Increase speed to medium and beat about 3 minutes or until mixture is smooth and changed to a paler colour.
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Pour mixture into pan; bake about 30 minutes. Stand cake in pan 5 minutes before turning, top-side up, onto wire rack to cool. Serve dusted with sifted cocoa powder, if you like.