Preheat oven to 400°F (200°C).

Unfold the puff pastry sheet on a lightly floured surface and roll it out to 1/8 inch thickness.

Cut the pastry into equal-sized squares and place onto a baking sheet lined with parchment paper.

Dock the pastry with a fork and bake for 12-15 minutes until golden brown. Let cool completely.

In a medium saucepan, whisk together sugar, cornstarch, and salt.

In a separate bowl, whisk egg yolks. Heat milk in a saucepan until simmering, then gradually pour into egg yolks while whisking.

Pour mixture back into saucepan and cook over medium heat, stirring constantly, until thickened. Remove from heat, stir in butter and vanilla extract.

Pour custard into a bowl, cover with plastic wrap pressed onto the surface, and refrigerate for at least 2 hours.

Spread a generous layer of chilled custard on one piece of baked puff pastry, top with another piece, and dust with powdered sugar.

Repeat until all squares are assembled. Serve immediately or store in the fridge.