Preheat oven to 400 °F (200 °C). Line two baking sheets with parchment paper and place each puff pastry sheet on a prepared sheet—prick all over with a fork to prevent excessive puffing. Bake for about 12–15 minutes, until golden and puffed. Let cool.

Meanwhile, prepare the pastry cream: in a saucepan, heat milk and half the sugar over medium until just simmering.

In a bowl, whisk egg yolks, remaining sugar, and cornstarch until smooth and pale.

Slowly pour about one-third of the hot milk into the yolk mixture, whisking constantly (tempering), then return the mixture to the saucepan. Cook over medium-low heat, whisking, until it thickens and begins to bubble—about 1–2 minutes. Remove from heat, whisk in butter and vanilla. Transfer to a bowl, cover surface with plastic wrap, and chill until completely cool.

Once pastry sheets and cream are ready, carefully layer: place one pastry sheet on a serving plate, spread half the pastry cream evenly, top with second pastry sheet, spread remaining cream, and finish with the top pastry. Dust with powdered sugar. Optionally chill for easier slicing and cleaner layers.

Slice carefully with a sharp knife, wiping the blade between cuts if needed for clean edges.