30 minutes, plus 2½ hours chilling
142ml carton double cream
115g caster sugar
11.7g sachet powdered gelatine
2 egg whites
Finely grate the lime zest and mix into the double cream. Chill until needed.
Squeeze the lime and pour the juice into a non-corrodible saucepan. Add the blackberries and sugar and set over a low heat to draw out the fruit's juices and dissolve the sugar, then cover and simmer gently for about 8 minutes or until the fruit is quite soft.
Meanwhile put 3 tbsp water in a small basin, add the gelatine and soak for about 5 minutes. Give it a stir and, if it has not dissolved, place the bowl in a pan of barely simmering water - it should melt within 2 minutes.
Take the cooked fruit off the heat, mix in the gelatine and push the mix through a sieve into a bowl. Leave until cool, then chill until it begins to set. This may take up to 2 hours but check it reguarly, as it will set fairly suddenly.
Lightly whip the lime-flavoured cream until it forms soft peaks. In a separate bowl, whisk the egg whites until they form soft peaks. Fold the cream into the setting blackberry purée, followed by the egg whites. Immediately spoon into a serving bowl and chill until set - about an hour. If you want four small mousses, use 150ml moulds, base-lining them with greaseproof paper if you want to turn them out for serving. Run a knife around the sides of the mousses before turning out.
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