Preheat oven to 450 degrees F. Line a baking sheet with parchment paper and set aside.
In a small bowl, whisk sugar and cornstarch until there are no more lumps from the cornstarch.
Add blueberries, sugar and cornstarch mixture, and lemon juice to a medium size saucepan.
Cook on medium heat stirring frequently until mixture is bubbling and thickened.
Once thickened, sprinkle cinnamon and stir into pie filling and set aside.
Roll out both pie crusts and lay them on top of each other. Cut out Christmas trees using a 2-inch cookie cutter.
Lay cut pie crust on the prepared baking sheet and add 2 teaspoons of pie filling to the middle of each tree.
Whisk egg to make egg wash, lightly brush the outside of the bottom crust with egg wash and lay the other on top.
Using a fork gently press together each side of the pie crust carefully to keep the Christmas tree shape.
Poke holes in the top of each pie, brush the tops with egg wash and sprinkle with sugar.
Bake pies for 5 min or until golden brown and then place on a cooling rack to cool.
While pies are cooling add powder sugar and milk to a bowl and whisk until desired consistency.
Drizzle cooled pies with icing.
Keep leftover pies in an airtight container in the refrigerator for up to 5 days.