Rinse and dry the fresh blueberries. Add them to a large bowl along with the cornstarch, granulated sugar, brown sugar, fresh lemon juice, vanilla extract, cinnamon, and salt. Stir until everything is evenly coated.
Remove the pie dish from the fridge or freezer and brush the inside of the dough with the egg wash. Use a slotted spoon to scoop the blueberry filling into the pie. Discard any excess liquid in the bowl. Dot the top of the filling all over with the pieces of butter.
Take the second portion of dough out of the freezer and use a sharp knife or a pastry wheel to cut it into ¾"-wide strips.*
To make the lattice topping: Arrange 5 strips of dough going in one direction across the top of the filling. Fold back every other strip halfway and place a strip of dough going the other way underneath them. Fold the strips back over and fold back the opposite strips. Continue folding back every other strip and adding a strip underneath until the entire pie is covered.
Trim the lattice so it's even with the bottom crust. Fold the edges under to create a thick rim around the outside of the pie. Make a fluted edge by pressing the rim in with one finger and then pinching it with your other two fingers, repeat this going all the way around.
Brush the top of the lattice with the egg wash and sprinkle with coarse or sanding sugar (optional).
Arrange your oven rack to the lower third position.
Place the pie in the freezer and begin preheating your oven to 400°F.
Set the pie on a large rimmed baking sheet and bake in the preheated oven for 20 minutes. Then, without taking the pie out of the oven, lower the temperature to 350°F and continue baking for 50-55 minutes. Cover the top of the pie loosely with aluminum foil about halfway through baking so it doesn't burn.
Take the pie out of the oven and allow it to cool for at least 3 hours before serving. The longer the pie cools the more set the filling will be.
Slice and serve with a side of vanilla ice cream. If you prefer the pie warm, you can microwave individual slices for about 30 seconds.