Step 1: Prepare the Pie Dough

• Mix dry ingredients: In a food processor, pulse together the flour, salt, and sugar.
• Add butter: Pulse in the cold butter cubes until the mixture resembles coarse crumbs.
• Add ice water: Slowly add ice water, 1 tablespoon at a time, pulsing until the dough comes together.
• Chill the dough: Divide into two discs, wrap in plastic, and refrigerate for at least 1 hour.

Step 2: Make the Blackberry Filling

• Mix the ingredients: In a large bowl, toss the blackberries with sugar, cornstarch, lemon juice, lemon zest, vanilla extract, and cinnamon. Let sit for 10 minutes to release juices.

Step 3: Roll Out the Dough

• Roll out the bottom crust: On a floured surface, roll out one dough disc into a 12-inch circle.
• Transfer to pie dish: Fit the dough into a 9-inch pie plate, trimming any excess overhang.

Step 4: Fill the Pie

• Add the blackberry mixture: Pour the filling into the crust, spreading evenly.
• Dot with butter: Distribute small butter pieces on top of the filling.

Step 5: Roll Out the Top Crust

• Prepare the top crust: Roll out the second dough disc and place it over the filling. Trim, tuck, and crimp the edges to seal.
• Create vent holes: Cut slits or use a lattice design for steam to escape.

Step 6: Brush with Egg Wash

• Mix egg wash: Whisk the egg yolk with heavy cream and brush over the top crust.

Step 7: Bake the Pie

• Preheat oven: Set the oven to 375°F (190°C).
• Bake until golden: Bake for 45–50 minutes, or until the crust is golden brown and the filling is bubbly.
• Cool before serving: Let the pie cool for at least 2 hours to allow the filling to set.

Step 8: Serve

• Slice and enjoy: Serve warm or at room temperature with vanilla ice cream or whipped cream.