Preheat the oven to 400°F (200°C). Place the puff pastry sheets on parchment-lined baking trays and prick them all over with a fork to reduce puffing.

Bake for 15 to 20 minutes until golden brown and crisp. Allow the pastry sheets to cool completely.

In a large bowl, whisk together the sugar, cornstarch, egg yolks, and 1 cup of the milk until smooth.

In a saucepan, heat the remaining milk over medium heat until hot but not boiling.

Slowly pour some of the hot milk into the egg mixture while whisking continuously. Return everything to the saucepan.

Cook over medium heat, whisking constantly, until the custard becomes very thick and smooth. Remove from heat and stir in the vanilla extract, butter, and salt.

Place one cooled puff pastry sheet into the bottom of an 8×8-inch baking dish.

Spread the warm custard evenly over the pastry, smoothing the surface with a spatula. Carefully place the second pastry sheet on top.

Cover the dish and refrigerate for at least 4 hours, or preferably overnight.

The custard needs time to fully set so the slices hold their shape when cut.

Dust the top generously with powdered sugar.

Using a sharp knife, cut into neat squares or rectangles and serve chilled. Clean the knife between cuts for the neatest presentation.