Make the Dough: In a large mixing bowl, combine the flour, salt, and granulated sugar. Whisk until well mixed. Add the cubed butter and use a pastry cutter or your fingers to work the butter into the flour until it resembles coarse crumbs.

Form the Dough: Gradually add ice water, one tablespoon at a time, mixing gently with a fork until the dough comes together. Shape the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Roll and Fold the Dough: After 30 minutes, roll the dough into a rectangle (about 1/4 inch thick). Fold the dough into thirds, like a letter. Wrap and refrigerate for another 30 minutes. Repeat the rolling and folding process two more times. After the final fold, refrigerate the dough for at least 1 hour or overnight.

Make the Cherry Filling (if homemade): In a medium saucepan over medium heat, combine cherries, ¼ cup sugar, cornstarch, and lemon juice. Cook for 5-7 minutes, stirring occasionally, until thickened. Remove from heat and let it cool completely.

Prepare the Pastry: Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Roll the dough out into a rectangle (1/4 inch thick), and cut into 4×4 inch squares or triangles. Make diagonal cuts from each corner towards the center (about 1 inch in).

Assemble the Pastries: Place a spoonful of the cherry filling in the center of each square. Fold the corners over the filling, overlapping them slightly. Brush the top with the beaten egg and milk for a golden finish.

Bake: Bake for 15-20 minutes, or until golden brown. Let cool slightly before dusting with powdered sugar.