Preheat your oven to 400°F (200°C). Roll out the puff pastry sheets onto a baking sheet lined with parchment paper. Bake in the oven for about 25 minutes or until golden brown. Allow the pastry to cool completely on a wire rack.

While the puff pastry cools, combine mixed berries in a bowl. Sprinkle with granulated sugar and cornstarch, gently tossing to coat. You may heat the mixture over medium-low heat for 5-7 minutes if you prefer warm berries.

In a large mixing bowl, pour in heavy whipping cream, followed by powdered sugar and vanilla extract. Whip on medium-high speed until soft peaks form, about 3-5 minutes.

Lay a piece of baked puff pastry on a serving platter, spread a layer of the whipped filling over it, followed by a layer of mixed berry filling. Repeat until all ingredients are used, finishing with whipped cream.

Cover the assembled cake with plastic wrap and refrigerate for at least 1 hour to allow the flavors to meld.