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1
Preheat oven to 210 C (410 F).
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2
Line a 9-inch square baking pan with aluminum foil, leaving some excess foil hanging over the sides so that the slice can be easily removed later.
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3
Line two baking sheets with baking paper and place one sheet of puff pastry on each. Bake for 9 minutes or until golden brown. Remove from the oven and leave to cool.
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4
Once cool, place one sheet of pastry in the bottom of the prepared 9-inch pan, trimming to fit if needed. Carefully cut the other sheet of pastry into 9 squares and set aside.
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5
Put milk in a small saucepan. Scrape out the seeds from the vanilla bean and place both seeds and the whole bean into the pot with the milk. Warm gently over medium heat, then once it's warm remove it from heat and set aside for 10 minutes.
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6
Place cornflour, custard powder and sugar in a large saucepan. Strain the vanilla milk into the pan, removing the vanilla bean, and whisk until smooth. Add cream to the pan and set over low heat, whisking constantly until the mixture thickens and boils. Make sure the custard has thickened sufficiently or it will not set properly in the fridge. Add butter and stir to combine. Remove the pan from heat and whisk in the egg yolks, one at a time, until smooth.
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7
Pour into the tin that you've lined with pastry and leave it to cool slightly. When the slice has cooled, arrange squares of the second pastry sheet on top of the custard mixture. Cover with cling wrap and refrigerate overnight.
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8
Remove the slice from pan and cut into squares, using the top sheet of pastry as a guide. Dust with powdered sugar before serving.
