Two sheets cold puff pastry (about 1/2 pound each), preferably all butter
All-purpose flour, for dusting
3 tablespoons heavy cream
3/4 cup raspberries
1 1/2 tablespoons sugar
3/4 cup heavy cream
1/3 cup mascarpone
Make the pies: Line 2 baking sheets with parchment paper. In a medium bowl, toss the blueberries, blackberries and raspberries with 1 1/2 tablespoons of the sugar and the cinnamon and salt. On a lightly floured work surface, roll out the puff pastry to 1/8 inch thick. Cut each pastry sheet into 4 squares and transfer them to the baking sheets. Spoon the berry filling over half of each pastry square, leaving a 1/2-inch border; fold the pastry over to create a triangle and press the edges together firmly to seal. Crimp the edges with a fork. Cut a small slit on the top of each pie. Refrigerate for at least 20 minutes.
Preheat the oven to 400°. Brush the tops of the pies with the cream and sprinkle with the remaining 2 tablespoons of sugar. Bake the pies until golden, about 40 minutes. Let cool for 10 minutes before serving.
Make the whipped cream: In a small bowl, using a fork, crush the raspberries with the sugar until they are broken down but still slightly chunky. In a medium bowl, beat the cream with the mascarpone until stiff. Gently fold the crushed raspberries into the whipped cream. Refrigerate until ready to serve.
Serve the hand pies warm or at room temperature with a generous dollop of the raspberry whipped cream.