Wash the blackberries gently under cold water. Pat them dry with a soft towel. Place them in a large bowl. Add sugar, cornstarch, lemon juice, cinnamon and salt. Mix everything carefully without crushing the berries too much.

Roll out one pie crust and place it into a greased pie dish. Press it gently into the edges. Make sure there are no air pockets. Keep the second crust aside for topping the pie later.

Pour the blackberry mixture into the prepared crust. Spread it evenly using a spoon. Add small pieces of butter on top of the filling. This helps create a rich and smooth flavor while baking.

Place the second pie crust over the filling. Trim the edges and seal them by pressing with a fork. Cut small slits on top to allow steam to escape. This helps the pie bake evenly.

Mix egg and milk in a small bowl. Brush this mixture over the top crust for a golden finish. Place the pie in a preheated oven at 375°F (190°C). Bake for 40–45 minutes until golden and bubbly.

Remove the pie from the oven and place it on a cooling rack. Let it cool for at least 30 minutes. This helps the filling set properly. Slice and serve warm or at room temperature.