Thaw and roll out the puff pastry sheets. Cut them to your desired cake size and bake them at 400°F (200°C) for approximately 20-25 minutes, or until golden brown. Allow them to cool completely on a wire rack.

In a medium saucepan, combine the mixed berries with granulated sugar and cornstarch. Gently simmer over medium heat, stirring until the mixture thickens—about 5-7 minutes. Remove from heat and let it cool.

In a large mixing bowl, beat the heavy whipping cream with powdered sugar and vanilla extract until soft peaks form.

Take one layer of the cooled pastry and spread a generous layer of the berry filling on top. Follow it up with a layer of whipped cream. Repeat this process, stacking the layers carefully until all ingredients are used.

Refrigerate the assembled cake for at least one hour to allow it to set and the flavors to meld beautifully.