Preheat your oven to 375°F (190°C).
In a large bowl, combine the blackberries, sugar, cornstarch, lemon juice, and vanilla extract.
Gently toss the mixture until the blackberries are well coated.
Roll out the bottom pie crust and place it in a pie dish.
Pour the blackberry mixture into the crust, spreading it evenly.
Dot the filling with small pieces of butter.
Roll out the top pie crust and place it over the filling.
Cut slits in the top crust to allow steam to escape.
Mix the egg and milk, then brush this egg wash over the top crust.
Bake in the preheated oven for 40-45 minutes, until the crust is golden and the filling is bubbling.