1.Preheat oven to 240°C. Grease 8cm x 26cm (3¼-inch x 10½-inch) bar cake pan and line with strip of baking paper extending over long sides of pan.
2.Place pastry sheet on oven tray. Bake about 15 minutes or until puffed, then allow to cool. Split pastry in half horizontally; remove and discard any uncooked pastry from centre. Flatten pastry pieces gently with hand; trim both to fit bar pan. Place top half of pastry in pan, top-side down.
3.Meanwhile, in a medium saucepan combine sugar, cornflour and custard powder. Gradually stir in milk. Stir over high heat until mixture boils and thickens. Reduce heat. Simmer, stirring, about 3 minutes or until custard is thick and smooth. Remove pan from heat. Stir in butter, egg yolk and extract.
4.Spread hot custard over the pastry in pan. Top with remaining pastry, bottom-side up. Press down gently. Cool to room temperature.
5.Meanwhile, to make passionfruit icing, sift icing sugar into small heatproof bowl and stir in passionfruit and enough water to make a thick paste. Stir over a small saucepan of simmering water until icing is spreadable.
6.Spread icing over pastry; set at room temperature. Refrigerate 3 hours before cutting.