Drain the sour cherries and collect the juice in a bowl. Take a little of the cherry juice and mix it with the cornstarch. Bring the remaining cherry juice to a boil in a saucepan and add the cherry juice-cornstarch mixture. Cook until thickened, then fold in the cherries and let the mixture cool.
Prepare the vanilla pudding mix with milk and sugar according to the package directions and let the finished pudding cool.
Whisk the egg in a bowl.
Preheat oven to 400°F. Spread the puff pastry on a baking sheet and cut into 8 even squares.
Place a spoonful of pudding and cherry mixture in the center of each square.
Fold the squares diagonally into triangles. Press the edges firmly with a fork. Brush the surface with the egg.
Bake your turnovers for 15-20 minutes, or until golden brown.
Once the cherry pastries have cooled slightly, melt the chocolate with the butter in a double boiler and brush the finished pastries with the glaze as desired.