Prepare the Filling:

1. Combine the Blueberries and Ingredients: In a large bowl, toss the blueberries with lemon juice, sugar, cornstarch, vanilla extract, and cinnamon (if using). Set aside to allow the juices to begin to release while you prepare the crust.

Prepare the Crust:

2. Preheat the Oven: Preheat your oven to 375°F (190°C).
3. Roll Out the Pie Dough: Lightly flour your work surface and roll out the pie dough to about ⅛-inch thickness. Cut the dough into 6 equal circles (about 5 inches in diameter each).
4. Place the Dough on a Baking Sheet: Arrange the dough circles on a parchment-lined baking sheet.

Assemble the Galettes:

5. Add the Filling: Spoon a generous amount of the blueberry filling into the center of each dough circle, leaving about a 1-inch border around the edges.
6. Fold the Edges: Gently fold the edges of the dough over the filling, creating a rustic, pleated look. The center should remain open, exposing the blueberries.
7. Add Butter: Dot the blueberry filling with small pieces of butter for extra richness.

Finish and Bake:

8. Apply the Egg Wash: In a small bowl, beat the egg and brush it over the exposed crust for a golden finish.
9. Sprinkle Sugar: Lightly sprinkle the crust with a bit of granulated sugar for added sweetness and crunch.
10. Bake the Galettes: Bake the galettes for 35-45 minutes, or until the crust is golden brown and the blueberry filling is bubbling.

Cool and Serve:

11. Cool: Allow the galettes to cool for about 10 minutes before serving. They can also be served warm or at room temperature.