Preheat oven to 375°F. Place pie plate or pie tin on a parchment lined baking tray.
Add bottom crust to pie plate.
In a large bowl, gently mix blackberries, sugar, cornstarch, and cinnamon. It is ok if some berries get crushed.
Add blackberry mixture to pie crust. Dot with butter pieces.
Cut slits or design in second pie crust (which allows steam to escape), or if preferred, create a lattice top. Place on top of the pie. Roll and crimp edges of pie crusts to seal, creating a pattern if desired (trim edges if needed).
Carefully brush the entire pie crust top with milk. Sprinkle with coarse sugar if desired. Place pie tin on parchment lined baking tray. Bake 45-55 minutes, until berries are bubbling and slightly thickened (internal temperature should reach 205°F). After about 25-30 minutes, and if the top starts to get browned, loosely tent with foil for the remainder of baking time.
Cool completely on a cooling rack (3-4 hours) before cutting and serving. Slice and serve with ice cream or whipped cream!
Notes
Line your tray with parchment for easy cleanup.
Adjust sugar to taste.
Vent crust to release steam.
Cool before slicing to prevent leaks.
Spoon out extra juice and use it in smoothies or over ice cream.
Keep leftovers in an airtight container in the refrigerator for 3 days or in the freezer for 3 months.