Preheat your oven to 400°F (200°C).

Roll out the puff pastry sheets on a lightly floured surface until they fit your baking tray.

Place one sheet of pastry on a baking tray lined with parchment paper. Prick the surface with a fork.

Bake the pastry for 15-20 minutes until golden brown and crisp.

Heat the milk in a saucepan over medium heat until it just begins to simmer, then remove from heat.

Whisk together the eggs, granulated sugar, and cornstarch in a mixing bowl until smooth.

Gradually pour the hot milk into the egg mixture, whisking continuously.

Return the mixture to the saucepan and cook over medium heat, stirring constantly for about 5-7 minutes until thickened.

Remove from heat and mix in vanilla extract and butter.

Transfer the custard to a bowl and cover with plastic wrap.

Set the first layer of puff pastry on a serving platter and spread half of the custard evenly over it.

Add the second layer of puff pastry on top and spread the remaining custard over it.

Optional: Place a third layer of puff pastry on top.

Mix powdered sugar with enough milk to achieve a smooth icing consistency.

Pour the icing over the top layer of pastry and smooth it out.

Decorate with chocolate or colored icing if desired.

Refrigerate the assembled slice for at least 2 hours before serving.

Slice into squares or rectangles and serve.