Preheat the oven to 425F. In a small bowl, stir together the blueberries, sugar, cornstarch, lemon juice, cinnamon and a pinch of salt. On a lightly floured surface, roll the puff pastry (it should still be cold) into a 10-inch square. Cut into four quarters and place each piece on a parchment-lined sheet.

Divide the blueberry mixture between the pastry, dot each with a bit of butter, and fold the edges of the pastry about 1/2-inch over on all sides to contain the berries. Brush the pastry with some beaten egg and sprinkle with sugar.

Bake for 20 minutes, or until bubbly and golden. Serve warm, with whipped cream sweetened with maple syrup.