Wrap a double thickness strip of nonstick baking paper around a 13 cm (5½ inch) diameter x 6 cm (2½ inch) deep soufflé dish so the paper stands 6 cm (2½ inches) above the dish top. Put the blackcurrants and 2 tablespoons of the water in a saucepan, cover and cook gently for 5 minutes until softened. Blend until smooth, then press through a sieve.
Put the remaining water in a small heatproof bowl and sprinkle over the gelatine, making sure the water absorbs all the powder. Set aside for 5 minutes, then stand the bowl in a pan half-filled with boiling water and simmer for 3–4 minutes, stirring occasionally, until the gelatine dissolves to a clear liquid.
Put the yolks and sugar in a large heatproof bowl and place over a pan of simmering water so the bowl's base is not touching the water. Whisk for 10 minutes or until the eggs are very thick and pale, and leave a trail when lifted above the mixture. Remove from the heat and continue whisking until cool. Fold in the dissolved gelatine in a thin, steady stream, then fold in the purée.
Whip the cream softly, then fold into the soufflé mix with the mint. Whisk the whites into stiff, moist-looking peaks. Fold a large spoonful into the soufflé mixture to loosen it, then gently fold in the remaining whites. Pour the mixture into the soufflé dish so that it stands above the rim of the dish. Chill for 4 hours or until set.
Remove the string and paper. Arrange 4–5 strips of nonstick baking paper over the soufflé top so some overlap, then dust with sifted icing sugar. Lift off the strips and serve immediately or the sugar will dissolve.