Place a rack in the middle of the oven and preheat it to 220 °C / 430 °F or preheat the fan-assisted oven to 210 °C / 410 °F. Line a large 30 cm x 40 cm (12- inch x 16-inch) baking sheet with parchment paper.
bake the puff pastry
Unroll the puff pastry and place it on the prepared baking sheet. Using a fork, prick the dough all over. Place a sheet of parchment paper on top of the dough and place a rack over the parchment paper. Place in the oven and bake for 12 - 15 minutes at 220 °C / 430 °F (fan-assisted oven 210 °C / 410 °F) or until golden brown and baked. Transfer the baked puff pastry to a wire rack and repeat the process with another sheet of puff pastry. Using a knife, cut the baked puff pastry into the size of the Custard Slice baking dish (approx. 20 cm x 30 cm or 8-inch x 12-inch). Place one baked puff pastry dough into the Custard Slice baking dish.
Separate the egg whites and egg yolks. Add the egg whites to a large bowl and the egg yolks to a separate glass or metal bowl. Add sugar, flour, cornstarch, vanilla, and 80ml (1/3 cup) cold milk (take the milk from the liter needed) to the egg yolks. Using an electric mixer or a whisk, beat the egg yolks until pale, for about 2 minutes.
milk and egg whites
Pour the rest of the milk into a heavy-bottom saucepan. Add the vanilla and place over medium-high heat. Bring to a boil. While the milk is heating, beat the egg whites with the sugar until stiff peaks form.
cook the custard
Gradually pour the boiling hot milk into the egg yolk mixture while beating with an electric mixer. Mix until combined, then pour the mixture back into the saucepan and place over low heat. When the cream thickens, cook for another 6 - 8 minutes or until very thick and creamy. It's important to stir continuously while cooking, especially at the bottom of the pan, to prevent any lumps from forming. You can use an electric mixer or a whisk.
Remove the custard from the heat and whisk well. Add a third of the egg whites to the hot custard in the pan. Stir well using a spatula, then add half of the custard to the egg whites. Stir using a spatula to get a smooth mixture. Add the rest of the custard and stir to combine.TipThis whole process shouldn't last more than 2 minutes, or you can get lumps and scrambled eggs texture.
layering - custard and pastry
Pour the custard over the baked puff pastry in the pan and arrange evenly using a spatula. Place the custard in the fridge for 2 - 3 hours to cool and set.
layering - whipped cream
Make the Chantilly cream. In a bowl, whisk the whipping cream with the sugar until stiff peaks form. Spread the cream on top of the custard. Place the second baked puff pastry on top and place in the fridge for at least 3 hours or, even better, overnight.
The easiest way to cut these Custard Slices is to use a bread knife to cut through the upper layer of puff pastry. Cut twelve 7 cm x 7 cm (3-inch x 3-inch) slices. Then use a chef's knife to cut through the rest of the slices. Using a spatula, transfer each Custard Slice to a serving plate. Sprinkle with powdered sugar and serve.