Crisp, flaky layers of puff pastry sandwich clouds of vanilla crème légère, finished with a classic marbled vanilla-and-chocolate icing for an elegant, patisserie-style dessert.
Make pastry cream: Whisk egg yolks and cornstarch together in a heatproof bowl until smooth.
Heat milk and granulated sugar in a saucepan until just simmering; remove from heat.
Temper the yolk mixture by slowly whisking in the hot milk mixture in a thin stream.
Strain the mixture back into the saucepan through a fine-mesh sieve to remove any solids.
Cook over medium heat, whisking constantly, until it reaches a full boil and thickens.
Remove from heat; whisk in butter, vanilla extract, vanilla bean (if using), and a pinch of salt.
Transfer to a bowl, press plastic wrap directly on the surface, and chill for at least 3 hours or overnight.
Whip heavy cream to soft peaks, then fold into the chilled pastry cream to make crème légère; chill 1 hour.
Prepare puff pastry: Roll thawed store-bought puff pastry or chilled homemade rough puff to an even rectangle; transfer to a lined baking sheet and dock all over with a fork.
Place parchment over the pastry, set a second baking sheet on top, and weigh with pie weights or dried beans; bake 20 minutes.
Remove top sheet and weights; continue baking uncovered 7–9 minutes until deeply golden; cool completely.
Trim edges and cut pastry into 15 equal squares to yield 5 pastries of 3 layers each.
Ice the tops: Place 5 pastry squares on a rack, spread each with thick vanilla icing.
Pipe or drizzle melted dark/semi-sweet chocolate in stripes, then drag a toothpick perpendicularly to create a chevron pattern; let set.
Assemble: Place one plain pastry square on a board and pipe 9 dollops of crème légère (a 3-by-3 grid).
Top with a second plain square; repeat piping 9 dollops of cream.
Finish with a decorated, iced top layer.
Refrigerate assembled pastries, uncovered, for at least 30 minutes to set before serving.
Serve chilled and enjoy.