Melt the chocolate in a bowl on a medium setting in the microwave, stirring every minute to ensure that it melts evenly. Soak the gelatin leaves in cold water, until soft, then dissolve by warming through over a low heat and stirring. Whip the cream until it forms ribbon shapes when the whisk passes through it. Separate the eggs and whisk the yolk until it begins to whiten.
With an electric beater whip the egg whites until they begin to, first whiten, then stiffen and slowly add the caster sugar. Continue whipping until the sugar is dissolved. In a large bowl place the egg yolks, cream, egg white and gelatin. Gently fold over a few times, then add the chocolate. Fold everything together quite quickly, as the chocolate will stiffen as it comes in to contact with the colder ingredients. Pour into a cake tin with detachable sides that has grease proof paper on the bottom. Set in the fridge for 6-8 hours then, with a small hot wet knife, cut carefully around the edge of the tin, re-wetting the knife so it moves smoothly, and remove the outer.