1.In a medium saucepan, blend custard powder to a smooth paste with about ½ cup of the milk. Stir in caster sugar and remaining milk. Place over heat, stirring constantly, until custard boils and thickens. Remove from heat, stir in vanilla essence, then transfer custard to a large bowl. Cover bowl with plastic wrap and cool.
2.Beat cream until soft peaks form. Using an electric mixer or a wire whisk, beat the whipped cream into custard in two lots until custard is smooth and thick. Cover custard and refrigerate. Place each pastry sheet on a separate oven tray. Bake pastry sheets in a 220°C preheated oven 10-12 minutes or until golden brown.
3.Remove from oven and cool on trays. Lift one pastry square onto a clean chopping board, spread with half the chilled custard, then top with another pastry square and the remaining custard. Place remaining pastry square on top. Cover and refrigerate slice for at least 1 hour. Cut into rectangular pieces using a large serrated knife. Sieve tops with icing sugar, if desired.
Slice can be prepared several hours ahead. Keep covered in the refrigerator.Note