In a medium saucepan, combine chopped cherries, granulated sugar, cornstarch, vanilla extract, cinnamon (if using), and lemon juice. Cook over medium heat, stirring constantly, until the mixture thickens and cherries release their juice, about 5-7 minutes. Remove from heat and cool to room temperature.

1. Preheat your oven to 375°F (190°C). Roll out the puff pastry on a lightly floured surface into a rectangle large enough to wrap around the cherry filling.
2. Spoon the cooled cherry filling along the center of the pastry, leaving space on each side for folding. Fold the pastry sides over the filling, then roll it into a log or cylinder shape, sealing the edges.
3. Beat the egg and brush it over the top of the strudel for a golden finish.
4. Place the strudel seam-side down on a parchment-lined baking sheet. Brush the top with melted butter. Bake for 25-30 minutes or until the pastry is golden and puffed.
5. Let the strudel cool for a few minutes, then dust with powdered sugar before serving.