Line a 5x9 inch loaf pan with plastic wrap allowing a 4 inch overhang on all sides.
In a small bowl stir together the wafers, butter and salt, press evenly into the bottom of the pan and freeze.
In a double boiler, whisk together the egg yolks and sugar and cook, whisking until the sugar has dissolved, around 3 minutes. Slowly whisk in the citrus peel and juice over low heat until the mixture has thickened, around 10 to 12 minutes. Let cool for 10 minutes.
Using an electric mixer, beat the cream until soft peaks form. Fold into the custard.
Pour the filling over the crust. Fold the plastic wrap over the cake and freeze for at least 5 hours over overnight.
To serve run a wet knife around the sides of the pan and invert onto a platter, discarding plastic wrap,