1. Preheat oven to 400°F.
2. In a large bowl combine the blueberries, granulated sugar, cornstarch, lemon juice and Cinnamon. Set aside.
3. On a floured work surface, roll out one of the pie crusts to 12 inches diameter. Carefully transfer the pie dough into a 9x2 inch pie dish. Tuck it in with your fingers, making sure it is smooth.
4. Spoon the filling into the crust, leaving any excess liquid in the bowl (you don't want a soupy pie - discard it). Dot the filling with pieces of butter.
5. To make a lattice top: roll the remaining crust to 12 inches diameter. Using a sharp knife, or pizza cutter, cut strips 1-2 inches wide. Carefully thread the strips over and under one another, pulling back strips as necessary to weave. Press the edges of the strips into the bottom pie crust edges to seal. Use a small knife to trim off excess dough. Crimp the edges with a fork, if desired.
6. Lightly brush the egg + milk wash over the crust. Sprinkle the top with a little coarse sugar, if desired.
7. Place your pie on a rimmed baking sheet and refrigerate for 1 hour. (this will ensure your crust stays put while baking)
8. Bake for 20 minutes. Then reduce oven temperature to 350°F and bake for an additional 25-30 minutes or until crust is golden brown. Cool completely before serving.