In a medium bowl, combine 2 cups of mixed berries, 1/3 cup of sugar, and 2 tablespoons of cornstarch. Gently toss to coat the berries and let them sit while you prepare the pastry.
On a lightly floured surface, roll out the puff pastry sheet to smooth out any creases. If using two sheets, make sure they are the same size.
Cut the puff pastry into rectangles, approximately 4×6 inches.
Place a tablespoon of the berry mixture in the center of each pastry rectangle. Fold the pastry over the filling and pinch the edges to seal. Twist the edges slightly for a decorative touch.
Brush the tops of the twists with the egg wash for a shiny finish.
Arrange the twists on a parchment-lined baking sheet and bake for 15-20 minutes or until golden brown and puffed up.
Once baked, remove the Berrypufftwists from the oven and let them cool slightly before serving.