Heat the oven to 150°C, 130°C fan. Melt the butter and chocolate in a large bowl over a pan of barely-simmering water, stirring occasionally.
In a small pan, gently heat the sugar with 4tbsp water until dissolved. Increase the heat and bring to the boil for 1 minute. Stir through the chocolate mixture, remove from the heat, and allow to cool slightly. Stir through the egg yolks and set aside.
Whisk the egg whites to a medium peak and fold into the cooled chocolate mixture. Pour into the tin and bake on a baking tray for 50 minutes to 1 hour, until there is only a slight wobble in the centre.
Allow to cool in the tin overnight, and serve with whipped cream or vanilla ice cream.
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