Preheat the oven to 150°C (300°F). Place the frozen puff pastry sheets on two separate baking trays lined with parchment paper. Allow the pastry to thaw. Once thawed, prick the surface of each sheet with a fork. Cover with another piece of parchment paper and place a second baking tray on top. Bake for 20 minutes or until the pastry attains a golden color. After baking, let the pastry cool down completely. Place a square 9-inch baking tin on top of one pastry sheet and trim it to fit the tin's bottom using a serrated knife. Repeat this process for the second pastry sheet, trimming it to fit the top of the baking tin.

Combine the caster sugar and corn starch in a large, heat-proof bowl. Whisk them together. Incorporate the egg yolks into the mixture, whisking thoroughly. Gradually add 1/2 cup of the hot milk, whisking continuously. Then, incorporate the remaining milk and the vanilla bean paste, continuing to whisk.

Transfer the mixture to a large saucepan. Cook over medium-high heat, whisking constantly until the mixture thickens, approximately 5-6 minutes. Remove from heat and add the unsalted butter, whisking until the butter is completely melted and well incorporated into the custard.

Line the bottom and sides of the baking tin with parchment paper. Place the smaller pastry sheet inside the tin. Sieve the custard over the pastry, ensuring an even distribution. Smooth the custard surface using a spatula. Place the second pastry sheet on top of the custard. Refrigerate the assembled dessert overnight to allow it to firm up.

Carefully remove the vanilla slice from the baking tin and place it on a chopping board.

Use a sharp, serrated knife to score the top pastry layer gently. Then, slice through the entire dessert in a single motion, ensuring to wipe the knife clean between cuts. Conclude by dusting the top with powdered sugar before serving.