1. Preheat your oven to 375°F (190°C).

2. On a floured surface, roll out your pie dough about 2 inches larger than your tart pan.

3. Press the dough into the pan and trim any excess.

4. In a mixing bowl, combine berries, sugar, cornstarch, salt, vanilla extract, and lemon juice. Stir to mix well.

5. Pour the berry mixture into the dough-lined tart pan.

6. Fold over the edges of the dough onto the berries to create a rustic look.

7. Beat the egg and brush it over the exposed dough for a shiny finish.

8. Bake in the preheated oven for about 30-35 minutes, or until the crust is golden and the berries are bubbling.

9. Allow the tart to cool before dusting with powdered sugar, serving to mimic the magical wintery setting of the White Witch's territory.

10. Slice and serve your delicious Winterberry Tart, inspired by the Chronicles of Narnia. Enjoy!