Step 1Preheat oven to 160°C/140°C fan-forced. Grease a 6cm-deep, 22cm (base) round springform cake pan. Line base and side with baking paper.
Step 2Place butter and chocolate in a heatproof bowl over a pan of simmering water (make sure bowl doesn’t touch water). Stir over low heat until smooth.
Step 3Using an electric mixer, beat egg yolks and 1/2 cup sugar for 5 minutes or until thick and creamy. Beat eggwhites in a separate bowl until soft peaks form. Gradually beat in remaining sugar until dissolved.
Step 4Add chocolate mixture to egg yolk mixture. Mix to combine. Add one-quarter of the eggwhite mixture. Mix well. Fold in remaining eggwhite mixture. Pour into prepared pan. Bake for 50 minutes or until top is firm to touch and a skewer inserted into centre has moist crumbs clinging. Cool cake in pan. Serve.