Preheat the oven to 180°C (350°F). Grease a baking sheet with butter or line it with parchment paper.
In a bowl, mix 500 grams of pitted cherries, 100 grams of granulated sugar, 1 teaspoon of cinnamon powder, and 1 tablespoon of cornstarch. Set aside.
On a lightly floured surface, roll out 250 grams of puff pastry into a large rectangle, about 3-4 mm thick.
Spread the cherry mixture evenly over the pastry, leaving a 2 cm border around the edges.
Gently roll up the pastry from the longer side, sealing the edges by pressing them together with your fingers or a fork.
Carefully transfer the strudel to the prepared baking sheet, seam side down. Make small diagonal slits on the top of the strudel to allow steam to escape.
Brush the strudel with a beaten egg for a golden finish.
Bake for 35-40 minutes, or until the pastry is golden brown and the filling is bubbling.
Allow the strudel to cool slightly before dusting with powdered sugar and serving with a scoop of vanilla ice cream or whipped cream.