Cut the cold unsalted butter into small cubes and mix it with flour, granulated sugar, and salt in a large mixing bowl. Blend until coarse crumbs form. Gradually drizzle in the ice-cold water until a dough forms. Shape into a disk, wrap in plastic wrap, and chill in the refrigerator for at least 30 minutes.

While the dough chills, slice the firm plums into wedges and place them in a mixing bowl. Sprinkle with granulated sugar, cornstarch, and cinnamon, gently tossing to combine. Set aside to allow flavors to meld.

Flour your work surface and roll the chilled dough into a 14-inch round, ensuring even thickness. Transfer it to a parchment-lined baking sheet. Arrange the plum filling in the center, leaving a 2-inch border.

Fold the edges of the dough over the plums, drizzling remaining plum juices over filling, then brush the crust with the beaten egg. Optionally sprinkle with cinnamon sugar.

Preheat your oven to 425°F (220°C). Bake the assembled tart for 15 minutes, then reduce temperature to 350°F (175°C) and bake for an additional 20-30 minutes until the crust is golden brown and plums are tender.

Remove the tart from the oven and brush the warm plums with plum or peach jam for a glossy finish. Allow to cool on the baking sheet for at least 10 minutes before slicing. Serve warm or at room temperature.