2) In a medium bowl, whip the double cream, icing sugar and vanilla until stiff peaks form. Set aside.
3) In another medium bowl, whip egg whites until foamy. With the mixer on, slowly add the simple syrup. Continue beating on a high speed until the mixture is glossy and doubles in volume, about 7 to 10 minutes.
4) Using a large rubber spatula, fold the pineapple into the egg whites. Fold the whipped cream into the egg white mixture.
5) Gently, spoon the mixture into the loaf pan. Fold over the cling film to cover and freeze for 8 hours (or for up to 1 month).
6) To serve, unfold the cling film. Invert the semifreddo onto a cutting board and remove the cling film. Cut the semifreddo into 2 1/2cm thick slices. Place on individual serving plates and top each slice with the crushed amaretti cookies.
For the simple syrup:
1) In a saucepan, combine the water and sugar over a medium heat. Bring to the boil, reduce the heat and simmer for 5 minutes, until the sugar has dissolved.
2) Take the pan off the heat and cool the syrup. Any extra cooled syrup can be saved in an airtight container. Makes 100ml.