1️⃣ Bake the pastry:
Preheat oven to 400°F. Roll each sheet of puff pastry into a 10×12-inch rectangle. Place on parchment-lined baking sheets. Prick all over with a fork and cover with another sheet of parchment and a second baking sheet to weigh it down. Bake 15–20 minutes until golden and crisp. Cool completely.
2️⃣ Make the pastry cream:
Heat milk in a saucepan until steaming. In a bowl, whisk sugar, yolks, and cornstarch until smooth. Slowly whisk in hot milk, return to pan, and cook over medium heat, whisking constantly, until thickened (about 2 minutes). Off heat, stir in vanilla and butter. Cool, then refrigerate.
3️⃣ Assemble:
Trim pastry sheets to even rectangles. Place one on a serving plate, spread half the pastry cream, top with another sheet, repeat with cream, then top with final pastry sheet.
4️⃣ Glaze:
Mix powdered sugar with milk to make a thick glaze. Spread over top layer. Drizzle melted chocolate and drag a toothpick through to create marbling. Chill 30 minutes to set.
5️⃣ Slice gently with a serrated knife and serve!