In a small bowl, whisk the sour cream and egg yolk until combined. Set aside.
In the work bowl of a food processor, pulse the flour, sugar, and salt a few times to combine. Alternatively, whisk the ingredients together in a large mixing bowl.
Add the butter to the flour mixture and pulse about 10 times, until butter is in pea size pieces. Alternatively, use a pastry cutter to combine the ingredients. Do not over mix.
Add half of the sour cream and egg mixture to the bowl. Pulse about 5 times to combine, then add remaining mixture and pulse/process to fully combine. Alternatively, whisk or stir the sour cream/egg mixture into the flour mixture.
Gather the dough and form two equal sized balls. Wrap tightly in plastic wrap and refrigerate for 30 minutes.
While the pastry dough chills, make the blackberry pie filling.
Blackberry Pie Filling
In a large bowl, stir together blackberries, brown sugar, cornstarch, cinnamon, vanilla extract and salt. Let the filling sit for 10 minutes.
When pie crust is cold, preheat oven to 350℉.
Lightly flour your work surface. Using a rolling pin, roll one of the dough balls out to a diameter of 10 inches. Place the crust into a 9-inch pie pan, and gently press into place. Trim excess dough from the edge. Roll out the other ball of dough to a diameter of 10 inches.
Pour blackberry filling evenly into the bottom crust.
Gently place the top crust over the berries and press down gently so it lays flat against the filling. Flute the edges of the crust using your thumb and forefinger.
Using the tip of a sharp knife, cut a small slit in the center of the pie crust to allow the filling to vent.
Bake
Bake in preheated oven for 35-45 minutes. Check the pie after 35 minutes to be sure the edges aren’t over-browning. If it is browning too quickly, cover the crust with foil or a pie crust protector.
When the pie is fully baked, the internal temperature should be at least 200℉, and filling should not be jiggly.Allow the pie to cool for at least 2 hours before slicing.
Notes
For the flakiest pie crust, be sure all of the ingredients are very cold. Also, avoid over mixing the pastry dough.
To prevent a soggy bottom crust, set a foil-lined baking sheet or foil-lined pizza stone on the bottom rack of the oven. Allow it to preheat, then set the pie dish directly onto it to bake the pie.
Using a glass pie plate allows you to see the bottom of the crust to determine if it is thoroughly baked.
The filling will thicken as it cools. Allow the pie to cool for at least 2 hours before slicing. Otherwise, the filling will be too runny.