Combine the blackberries and sugar in large saucepan and bring to a boil. Mix the cornstarch with 2 tablespoons water and add it to the pan. Simmer until thick, about 5 minutes. Stir in the vanilla. Remove from the heat and cool slightly.
Fill the individual cast-iron pans with 3/4 cup filling each and set on a baking sheet lined with foil.
Take out the Perfect Pie Crust dough and separate into 6 even balls. Roll out the first one until about 1/8 inch thick. Cut out a circle from the dough slightly larger than the pan. Place the circle on top of a filled pan and press the edges slightly. Repeat with all the dough and pans. With a pastry brush, lightly wash the top of the pies with the egg wash. With a sharp knife, make small slits in the top of each pie to allow hot air to escape.
Bake until the crusts are golden brown, 20 to 25 minutes. (The berries will bubble over slightly; that's normal.) Allow to sit for at least 15 minutes before serving.
Serve each individual pie with a scoop of vanilla ice cream.
Perfect Pie Crust:
Mix the flour and salt in a bowl. Add the butter and vegetable shortening and work into the flour using a dough cutter until the mixture resembles tiny pebbles. Add the white vinegar, egg and 5 tablespoons cold water and stir until just combined. Place the dough in a plastic bag and chill until needed.
This recipe can also be done in 1 large cast-iron pan.