Place milk and cream in a saucepan over medium heat. Cook, stirring, for 10 minutes or until almost simmering (don't boil).
Using an electric mixer, beat yolks, sugar and vanilla until thick and creamy. Add cornflour and custard powder. Beat until smooth. Gradually beat in milk mixture. Return mixture to pan. Cook, stirring, over low heat, for 6 to 8 minutes or until mixture boils, thickens and coats the back of a wooden spoon. Remove from heat. Stir in butter until melted and smooth. Place in a bowl. Cool slightly. Cover surface with plastic wrap. Set aside for 1 hour to cool.
Preheat oven to 200°C/180°C fan-forced. Line 2 baking trays with baking paper. Place 1 pastry sheet on each prepared tray. Bake for 15 to 20 minutes or until puffed and golden. Cool for 5 minutes. Cover with a tea towel. Using hands, gently press pastry to flatten.
Lightly grease a 6cm-deep, 23cm (base) square cake pan. Line base and sides with baking paper, allowing 3cm overhang on all sides. Place 1 pastry sheet in prepared pan. Pour custard over pastry. Top with remaining pastry sheet. Cover. Refrigerate for 4 hours or until set.
Meanwhile, make icing Place icing sugar and butter in a bowl. Stir in passionfruit pulp until a spreadable consistency forms. Spread icing over slice. Refrigerate for 10 minutes or until set. Lift slice from pan. Cut into 16 squares. Serve. You will need about 2 passionfruit.