Make the pie crust: In a large mixing bowl, combine the flour, sugar, and salt. Add the cold, cubed butter and cut it into the dry ingredients using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, mixing until the dough starts to come together. Add just enough water to form a dough—do not overwork it. Turn the dough out onto a floured surface, divide into two halves, and shape each into a disk. Wrap in plastic wrap and refrigerate for at least 30 minutes.

1. Prepare the cherry filling: In a medium saucepan, combine the cherries, sugar, lemon juice, cornstarch, and vanilla extract. Cook over medium heat, stirring constantly, until the mixture thickens and bubbles (about 5-7 minutes). Once thickened, remove from heat and stir in the butter (if using). Let the filling cool to room temperature while you roll out the dough.
2. Assemble the mini pies: Preheat your oven to 375°F (190°C). Grease a muffin tin or mini tart pans. Roll out one disk of dough on a floured surface to about 1/8-inch thickness. Cut out circles of dough large enough to fit into the muffin tin. Press the dough circles into the muffin tin cups, making sure to line the bottoms and sides. Spoon about 2 tablespoons of the cooled cherry filling into each crust. Roll out the second disk of dough and cut it into strips or circles to create the top crust. You can use a lattice pattern or cover each pie with a full circle of dough. If using full circles, make a few slits in the dough to allow steam to escape.
3. Add the egg wash: Brush the tops of the mini pies with the beaten egg wash for a golden, shiny finish as they bake.
4. Bake: Bake the pies in the preheated oven for 20-25 minutes or until the crust is golden brown and the filling is bubbling. Keep an eye on them to ensure they don’t over-brown. You can cover the edges with aluminum foil if they’re getting too dark.
5. Cool and serve: Let the mini cherry pies cool in the muffin tin for about 10 minutes, then transfer them to a wire rack to cool completely. Serve warm or at room temperature.