01 -
Heat the oven to 400°F (200°C). Line two baking sheets with parchment paper and line an 8-inch square pan with foil or parchment, leaving overhang for lifting.
02 -
Roll each puff pastry sheet slightly larger than the pan, prick all over with a fork, and place on the prepared baking sheets. Cover each with parchment and a second baking sheet. Bake for 25-35 minutes, until deeply golden and crisp. Cool completely.
03 -
Use a serrated knife to trim both pastry sheets to fit the 8-inch square pan. Set one sheet in the bottom of the lined pan.
04 -
Warm the milk in a saucepan until steaming but not boiling. In a separate medium saucepan, whisk sugar, cornstarch, salt, and egg yolks until smooth.
05 -
Slowly whisk about 1 cup of warm milk into the yolk mixture. Whisk in the remaining milk, then cook over medium heat, whisking constantly, until thick and bubbling. Continue whisking for 1 minute.
06 -
Remove from the heat. Whisk in the vanilla and cold butter until the custard is glossy and smooth. Strain through a fine sieve if you want an extra-smooth filling.
07 -
Spread the warm custard evenly over the bottom pastry sheet. Place the second pastry sheet on top and press very gently. Cover and refrigerate for at least 4 hours, preferably overnight.
08 -
Lift the slab from the pan. Cut into squares with a serrated knife, wiping the blade between cuts. Dust with powdered sugar right before serving.