In a large bowl, stir together the blueberries, sugar, cornstarch, lemon juice, cinnamon, and sea salt until coated, set aside.

Roll out the second crust onto a clean work surface. Cut the crust into 1-inch strips, I like to use a ruler to help make straight lines. Set aside.

Take the pie plate out of the fridge and fill it with the blueberry filling. Dot with the butter if using. Take 5 of the pie crust strips and lay them out parallel to each other with a small gap on top of the pie. You want to use the longer strips in the center of the pie and the shorter ones towards the edge. Take every other strip and fold it up over itself so the filling is exposed under those pieces. Lay 1 strip of pie crust on top. Fold the strips back over so you now have part of a weave. Do this process again but with the remaining strips. Repeat until the entire pie has lattice. Refer to the tutorial for detailed visual instructions.

Tuck the edges of the strips under the edge of the bottom pie crust. Crimp the edges around the entire pie.

In a small bowl, whisk the egg with the milk. Brush this mixture over the exposed pie crust. Sprinkle the sugar on top of the crust.