Preheat the oven to 375°F (190°C) and grease a 12-cup muffin tin.

Combine the blueberries, sugar, cornstarch, lemon juice, lemon zest, and vanilla extract in a medium bowl. Stir until the blueberries are well coated.

Roll out the pie crusts on a lightly floured surface.

Using a 4-inch round cookie cutter, cut out 12 circles from the dough.

Press each dough circle into the prepared muffin tin, ensuring they come up the sides to form a mini pie shell.

Fill each pie shell with the blueberry mixture.

Roll out the remaining dough scraps and cut into thin strips.

Create a lattice pattern over each mini pie.

Whisk the egg and water to make an egg wash in a small bowl.

Brush the top of each pie with the egg wash.

Bake for 18-20 minutes, or until the crust is golden brown and the filling is bubbly.

Allow the pies to cool in the tin for 10 minutes before transferring to a wire rack to cool completely.