1. Preheat oven to 400°F (200°C). Cut puff pastry sheets into equal rectangles. Place on parchment-lined baking sheets, prick with a fork, and cover with another sheet of parchment and a second tray. Bake for 12–15 minutes until golden. Cool completely.

2. In a saucepan, heat the milk until steaming. In a bowl, whisk egg yolks, sugar, and cornstarch until smooth.

3. Slowly pour the hot milk into the egg mixture while whisking, then return to the pan. Cook over medium heat, stirring constantly until thickened (about 5–7 minutes).

4. Remove from heat. Stir in butter and vanilla extract. Transfer to a bowl, cover with plastic wrap (touching the surface), and chill until fully cooled.

5. To assemble, place one pastry sheet on a serving dish. Spread a layer of custard. Repeat with the second layer and custard, then top with the third pastry.

6. Dust the top with powdered sugar. Garnish with fresh berries if desired.

7. Chill assembled cake for at least 2 hours before slicing for best texture and clean layers.