Preheat your oven to 400°F.
Arrange the 18 pieces of puff pastry on two baking sheets. Prick each square all over with a fork.
Bake 12-15 minutes, or until lightly browned.
While the puff pastry is cooking, make the blueberry sauce by adding the blueberries, water, sugar, cornstarch mixture, vanilla, and lemon juice to a saucepan over medium-high heat.
Bring to a boil, then lower the heat and simmer 3-5 minutes until thickened, stirring occasionally. Allow to cool some.
Once the puff pastry is cool, spread about 1 tablespoon of whipped cream onto 12 of the pastry pieces.
Lay strawberries over 6 of the whipped cream covered pieces, and raspberries over the other 6. Add another dollop of whipped cream on top of each layer of berries.
To assemble, place the strawberry pastry on top of the raspberry pastry then top with one of the remaining plain pastry pieces.
Place a spoonful of the blueberry sauce over each one and add more whipped cream if desired.