Line a baking sheet with parchment paper. Place defrosted puff pastry sheets (use two trays, if necessary) on baking sheet and bake for 15 to 18 minutes, or until golden brown. Cool completely on a wire rack.
While pastry is cooling, line a 9-inch square baking pan with aluminum foil or saran wrap, making sure the foil or wrap comes up over the sides. Place one sheet of pasty at the bottom of the pan, trimming the corners if necessary to make it fit the pan. Press down gently until pastry is relatively flat.
Pour the milk into a medium saucepan. Cut the vanilla bean in half and add it to the milk. Bring milk up to a simmer, then remove from heat. Scrap the seeds from the vanilla bean into the milk.
In a medium bowl, whisk together half n’ half, sugar, cornstarch, custard powder or pudding mix, and egg yolks until very smooth. Pour into the saucepan with the vanilla milk, whisking to combine. Turn up heat to medium and, whisking frequently, cook until mixture comes to a boil and is very thick. Remove from heat. Stir in vanilla extract and butter, continuing to stir until butter melts. Pour into prepared pan on top of the sheet of pastry and smooth into an even layer.
Gently press remaining sheet of pastry – corners trimmed if necessary – on top of the custard layer. Refrigerate at least 6 hours or overnight.
Slice a knife around the sides of the pan (to loosen the custard from the edges of the pan), then use the overhanging foil or saran wrap to easily lift the dessert from the pan. Dust generously with confectioners’ sugar before slicing.