Soak the gelatin leaves in cold water. Beat the egg yolks together with 50g sugar and zest of 2 lemons over a bain marie. Mix until the mixture is pale and creamy. Warm up a couple of spoons of cream and mix in the gelatin leaves until melted. Pass the ricotta through a fine sieve to obtain a smooth consistency. Squeeze the juice of the grated lemons and add to the yolk mixture together with the ricotta and gelatin. Whip the egg whites with the remaining sugar until it forms stiff peaks. Fold the cream in the ricotta mixture and fold in the meringue. Pour the mixture into desired moulds and freeze for 3 hours before serving.