Preheat your oven to 150°C (300°F). Thaw the puff pastry sheets until soft. Fork-pierce the sheets to prevent bubbling while baking. Place one sheet on a baking tray, cover with parchment paper, and weigh it down with another tray. Bake for 20 minutes until golden brown, then remove and allow to cool completely on a wire rack.

Once cooled, take the baked puff pastry sheets and cut one to fit the bottom of a 9-inch square baking tin. The second sheet will serve as the top layer, so trim it accordingly.

In a heat-proof bowl, whisk together the caster sugar and corn starch. Add the egg yolks and mix until combined. Gradually whisk in the hot milk, ensuring no lumps form. Cook on medium-high heat for about 5-6 minutes, stirring constantly until thickened. Remove from heat and stir in the unsalted butter until melted and smooth.

Line the baking tin with parchment paper, placing the trimmed pastry at the bottom. Pour the creamy custard over the pastry, leveling it with a spatula for an even surface. Lay the second pastry sheet on top, pressing gently. Chill in the refrigerator overnight.

Once chilled, remove the slice from the tin and peel away the parchment. Score the top layer with a knife and cut into even pieces for serving. Dust with powdered sugar before serving.