1.Preheat oven to 210°C or 190°C fan forced. Grease and line base and sides of a 23cm (base measurement) square cake pan with baking paper, extending paper at two opposite sides for handles. Line 3 baking trays with baking paper. Place 1 sheet of pastry on each prepared tray. Bake pastry, one tray at a time, for 8 minutes or until puffed and golden. Cover pastry with a clean tea-towel; using hands, press down pastry to flatten. Set aside for 10 minutes to cool.
2.Meanwhile, combine caster sugar, cornflour and custard powder in a large heavy-based saucepan. Gradually add milk, whisking until smooth. Stir in cream. Cook and stir over moderate heat for 15 minutes or until mixture thickens. Stir in butter and essence. Remove from heat. Whisk in egg yolks, one by one, until smooth.
3.To assemble slice, place a pastry sheet over base of prepared pan, trimming to fit. Spoon half the custard over pastry; level surface. Repeat layering. Top with remaining pastry, trimming to fit. Cover with plastic food wrap; chill for 6 hours or overnight until firm.
4.Transfer slice to a large flat plate or chopping board. Combine icing sugar, extra butter and passionfruit pulp in a bowl. Spread icing over pastry; let set. Cut slice into 16 squares. Serve.
Make ahead: Cook slice up to 4 days ahead. Store in an airtight container in the fridge.Note