Line a 20 cm (8 inch) shallow square cake tin with clingfilm. Arrange half of the trifle sponges in a single layer in the base of the tin.
Beat the butter, sugar and lemon rind together until pale and creamy. Gradually whisk in the egg yolks.
Whisk the egg whites in a large clean bowl until stiff, then whip the cream in a separate bowl. Fold the whipped cream then the egg whites into the creamed mixture. Gradually fold in the juice of ½ lemon.
Drizzle a little of the remaining lemon juice over the trifle sponges. Spoon the cream mixture on top and gently spread the surface level. Cover with a second layer of sponge slices, press them gently into the cream mixture and drizzle with the remaining lemon juice. Cover with an extra piece of clingfilm and chill in the refrigerator for 4 hours or overnight.
Remove the top layer of clingfilm, invert the cake on to a chopping board and peel off the remaining clingfilm. Decorate with berries and mint leaves, dusting the tops with icing sugar. Eat within 2 days of making, store in the refrigerator.