Preheat oven to 220°C/425°F. Grease 24cm round springform tin. Line oven trays with baking paper.
Place pastry sheets on oven trays; brush with a little water. Sprinkle evenly with sifted icing sugar. Sprinkle one pastry sheet with nuts. Bake about 15 minutes or until browned lightly. Gently flatten pastry with egg slice. Cut each sheet into 24cm rounds. Place plain sheet in tin.
Meanwhile, combine caster sugar, cornflour and custard powder in medium saucepan. Combine cream and milk in large jug. Gradually stir milk mixture into sugar mixture, until smooth.
Halve vanilla bean lengthways, scrape seeds into pan; add butter. Bring to the boil over medium heat, stirring constantly until mixture boils and thickens. Simmer for 3 minutes, stirring constantly. Remove from heat; stir in egg yolks. Spread mixture over pastry in tin. Top with remaining pastry round, nut-side up. Refrigerate 2 hours or until firm.
Serve dusted with a little extra sifted icing sugar.