Whisk egg whites with caster sugar until stiff peaks form.
Whisk in sieved icing sugar until the mixture is smooth and shiny.
Cut four discs from the sponge flan case using a 7.5cm cutter.
Place sponge discs on plates and drizzle with passion fruit liqueur.
Top each sponge disc with a large scoop of ice cream.
Pipe meringue mixture over the ice cream to cover it completely.
Brown the meringue using a blow torch or a very hot oven.
Simmer water and caster sugar in a pan until sugar dissolves.
Whisk in passion fruit purée and arrowroot and cook until thickened.
Remove from heat and stir in the fresh passion fruit seeds.
Serve by spooning the sauce around the base of each baked Alaska.