Whisk the egg whites with the caster sugar in a clean bowl until the mixture forms stiff peaks.
Add the icing sugar and whisk again until the mixture forms stiff peaks and is smooth and shiny.
Using a 7.5cm/3in cookie cutter, cut out four sponge discs.
Place the sponges onto serving plates and spoon over the passion fruit liqueur.
Top each one with a large scoop of ice cream.
Transfer the meringue mixture into a piping bag and pipe the meringue around the ice cream, covering from top to bottom.
Glaze the meringue with a blow torch, or briefly in a hot oven.
For the passion fruit sauce, place the water and caster sugar into a saucepan and heat until the sugar has melted and the mixture is just simmering.
Add the passion fruit purÃ©e and mix well then whisk in the arrowroot and cook until just thickened.
Remove from the heat and stir in the seeds from the halved passion fruit.
To serve, spoon the passion fruit sauce around the edge of each individual baked Alaska.