Preheat the oven with a baking sheet inside to 190°C, gas 5. Grease six 250ml soufflé dishes with the melted butter.
Mix 15ml (1tbsp) of the caster sugar in a bowl. Tip the mixture into each of the dishes in turn, rotating them so that they are evenly coated. Mix the cocoa with the coffee and rum.
Whisk the egg whites in a clean, grease-free bowl until they form firm peaks. Whisk in the remaining caster sugar. Stir a generous spoonful of the whites into the cocoa mixture to lighten it, then gently fold in the remaining whites.
Spoon the mixture into the prepared dishes, smoothing the tops. Place on the hot baking sheet, and bake for 12-15 minutes or until well risen.
Serve the soufflés immediately, lightly dusted with snow icing sugar.
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