PASTRY CREAM FILLING
8 large egg yolks (112 g.)
1/2 c. (56 g.) cornstarch
1/2 tsp. kosher salt
1 c. (198 g.) granulated sugar, divided
4 c. (946 mL) whole milk
4 tbsp. (1/2 stick; 57 g.) butter, room temperature
4 tsp. vanilla extract
2 c. (473 mL) heavy cream
PUFF PASTRY LAYERS
2 (1-lb.) boxes puff pastry, thawed
All-purpose flour, for rolling
ASSEMBLY
Mixed fresh berries, for serving
PASTRY CREAM FILLING
Step 1
In a large heatproof bowl, whisk egg yolks, cornstarch, salt, and 1/2 cup (99 g.) granulated sugar.
Step 2
In a medium pot over medium heat, heat milk and remaining 1/2 cup (98 g.) granulated sugar, stirring, until sugar is dissolved and milk is steaming. Whisking constantly, pour half of hot sugar milk into egg mixture. Pour milk-egg mixture into pot and cook over medium heat, stirring constantly, until pastry cream boils and thickens to a pudding-like consistency, 1 to 2 minutes. Cook, stirring, 1 minute more, then remove from heat. Stir in butter and vanilla.
Step 3
Pour hot pastry cream onto a rimmed baking sheet and cover with plastic wrap, pressing directly onto pastry cream. Refrigerate until cold.
Step 4
In a large bowl, whisk heavy cream until stiff peaks form. Transfer cold pastry cream to another large bowl and stir until smooth. Stir one-quarter of whipped cream into pastry cream until combined. Using a rubber spatula, carefully fold in remaining whipped cream. Refrigerate pastry cream filling until ready to assemble.
Step 5
Make Ahead: Pastry cream (without whipped cream folded in) can be made 3 days ahead. Keep chilled.
PUFF PASTRY LAYERS
Step 1
Preheat oven to 400° and line a rimmed baking sheet with parchment paper. Remove 1 sheet of puff pastry from refrigerator. On a lightly floured surface, roll out to a 13"-by-13" square. Cut square into 2 rectangles, transfer to prepared baking sheet, and prick all over with a fork.
Step 2
Bake, rotating sheet and flipping pastry halfway through, until deeply browned and crisp, 12 to 15 minutes. Transfer to a wire rack and let cool completely.
Step 3
Repeat with remaining 3 sheets of puff pastry for a total of 8 pastry rectangles.
ASSEMBLY
Step 1
Stack 7 puff pastry rectangles on top of one another; set remaining 1 rectangle aside. Using a serrated knife, trim edges so rectangles are a uniform shape; reserve crumbs.
Step 2
Arrange 1 puff pastry rectangle on a platter. Spread 1 cup pastry cream filling over puff pastry rectangle; top with a second rectangle and spread with 1 cup filling. Repeat layering with 5 remaining rectangles and 5 cups filling. Cover sides of cake with remaining filling.
Step 3
Loosely cover with plastic wrap and refrigerate cake at least 4 hours or, preferably, up to overnight.
Step 4
Before serving, crush reserved puff pastry rectangle to fine crumbs, add to reserved crumbs, then press into sides and top of cake. Top with berries.