How to Make It
Preheat the oven to 350°. In an 8-inch glass or ceramic baking dish, mix the cherries with the cornstarch, granulated sugar, lemon juice, 1/2 teaspoon of the lemon zest, 1/4 teaspoon of the vanilla and a pinch of salt. Bake for about 35 minutes, until the juices are thickened and bubbling. Drain the cherries, reserving the juices for another use. Chill the cherries.
In a medium bowl, whisk the butter with the confectioners' sugar until smooth. Whisk in the egg, then whisk in the 2 tablespoons of flour, the remaining 1 teaspoon of lemon zest and 1/4 teaspoon of vanilla; whisk in a pinch of salt. Chill until firm.
Line a large baking sheet with parchment paper. On a lightly floured work surface, roll out the puff pastry to a 15-by-20-inch rectangle, about 1/8 inch thick. Cut the puff pastry into twelve 5-inch squares and transfer them to a plate. Refrigerate for 10 minutes.
Arrange 3 pastry squares at a time on a work surface, keeping the rest chilled. Brush 2 adjoining edges of each square with the egg wash. Dollop a tablespoon of the lemon filling in the center of each square and top with 7 cherries. Fold the pastry over the filling to form a triangle, pressing out the air as you go. Press the edges firmly and crimp with a fork. Transfer the turnovers to the parchment-lined baking sheet. Repeat with the remaining pastry, lemon filling and sour cherries. Refrigerate the turnovers for about 15 minutes, or until firm.
Heat the oven to 400°. Using a sharp knife, trim the edges of the turnovers slightly. Brush the top of each turnover with egg wash and sprinkle with pearl sugar. Cut 2 or 3 small slashes in the tops to allow steam to escape. Bake for about 40 minutes, until the pastry is deep golden brown and the filling is bubbling out of the vents. Let the turnovers cool slightly, then serve with vanilla ice cream.
Friske Orchards (friske.com) sells excellent frozen sour cherries. Jarred, pitted sour cherries may also be substituted.