Pre-heat the oven to 400 degrees. Line a baking sheet with parchment paper and set aside.
In a mixing bowl, combine the berries, sugar, cornstarch and salt and stir together to combine.
On a clean, dry cutting board sprinkle a small amount of flour to prevent sticking and lay down your sheet of defrosted puff pastry. Using a rolling pin, roll the pastry into a 10”x14” rectangle of even thickness. Using a knife (or a pizza cutter works wonderfully for this!), cut the dough into 12 equal pieces (3 sections lengthwise and 4 widthwise). Place the pastry sections a little bit apart on your parchment lined baking sheet.
Fill a small dish with water. Dip your finger in the water and run it along the edges and corners of one of the pastry squares. Use your fingers to pinch the corners together, raising the edges to form little walls surrounding the inside of the pastry square (see photo). Repeat with all remaining pastry squares.
Divide the berry mixture amongst the pastry squares and place inside the pastry walls. Place the baking sheet with the pastries into the fridge (or freezer) for 10 minutes to chill (puff pastry puffs better when it goes into the oven cold).
Transfer the baking sheet into the preheated oven and bake for 20 minutes until the pastry is golden and puffy. Remove from the oven and sprinkle/dust the pastries with the powdered sugar.