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Put the orange in a microwave safe bowl.
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Add 250ml water, cover with clingfilm and microwave on high for 20 minutes, turning halfway through (or simmer for 1 hour in a small saucepan).
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Leave to cool, still covered.
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Heat oven to 180°C.
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Line a 20cm round springform tin with baking parchment.
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Cut the orange in half and remove pips.
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Put in a food processor with 5 tbsp of orange liquid left in the bowl and blitz to a smooth puree.
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Add the sugar, melted chocolate, egg yolks and baking powder and whiz again to mix thoroughly.
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Tip into a large bowl.
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Beat the egg whites until stiff, but not dry and fold into chocolate mixture.
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Spoon into lined tin.
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Put the tin on a baking sheet, then in the oven.
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Bake for 1 hour covering with a piece of foil or baking parchment halfway through to stop the top burning.
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Cool in the tin.