Whisk the flour, sugar, and salt together. Cut in the cold butter until coarse crumbs form with some pea-sized butter pieces remaining, then add ice water 1 tablespoon at a time until the dough holds together. Divide into two disks, wrap, and refrigerate for at least 1 hour.

Combine the blueberries, sugar, cornstarch, lemon juice, lemon zest, optional cinnamon, and salt. Toss gently until evenly coated and let the mixture stand for about 10 minutes.

Roll one chilled pastry disk into an approximately 12-inch round and fit it into a 9-inch pie plate. Add the blueberry filling and dot the top with the 2 tablespoons of butter.

Roll the second pastry disk and place it over the filling, or cut it into strips for a lattice crust. Trim and crimp the edges, then cut several vents if using a solid top crust. Chill the assembled pie for 15–30 minutes.

Whisk the egg with the milk or water and brush it over the crust. Sprinkle with the optional sugar, then bake at 425°F (220°C) for 15 minutes.

Reduce the oven temperature to 375°F (190°C) and bake for another 35–45 minutes, until the crust is deeply golden and the filling is visibly bubbling in the center.

Transfer the pie to a rack and cool for at least 3 hours before slicing. The filling thickens as it cools, giving each slice a clean, spoonable texture.