Preheat oven to 240C/475F. Grease a deep 23cm (9~inch) square cake pan; line with foil, extending the foil 10cm (4 inches) over the sides of the pan.
Place each pastry sheet on separate greased oven trays. Bake for 15 minutes; cool. Flatten pastry with hand. Place one pastry sheet in the pan; trim to fit.
Meanwhile, combine sugar, corn flour and custard powder in a medium saucepan; gradually add milk, stirring until smooth. Add butter; stir over heat until mixture boils and thickens. Simmer, stirring, for about 3 minutes or until custard is thick and smooth. Remove from heat; stir in egg yolk and vanilla. Cover surface of custard with plastic wrap; cool to room temperature.
Make passionfruit icing. Place sifted icing sugar, butter and pulp in a small heatproof bowl over a small saucepan of simmering water; stir mixture until icing is spreadable.
Whip cream until firm peaks form. Fold cream into custard, in two batches. Spread custard mixture over the pastry in the pan. Top with remaining pastry and trim to fit, if necessary; press down slightly. Spread pastry with the icing; refrigerate for 3 hours or overnight.