Steps
1. Preheat oven to 200°C (390°F). Roll out puff pastry to about 3mm thickness and cut into three equal rectangles.
2. Place pastry on a baking sheet lined with parchment paper, prick with a fork to prevent puffing excessively, and bake for 12-15 minutes until golden and crisp. Let cool.
3. To make pastry cream: Heat milk with a few drops of vanilla extract until just simmering.
4. In a bowl, whisk egg yolk, sugar, and cornstarch until smooth.
5. Slowly pour hot milk into egg mixture while whisking continuously, then return to saucepan.
6. Cook over low heat, stirring constantly until thickened. Remove from heat, stir in butter, and cool.
7. Whip heavy cream with powdered sugar until soft peaks form and fold gently into cooled pastry cream.
8. To assemble: Place one puff pastry layer on a plate, spread half the cream, top with another pastry, spread remaining cream, and finish with the final pastry layer.
9. For glaze: Melt butter, mix with powdered sugar, and lightly brush over the top layer.
10. Chill for 1 hour before serving for best flavors.