Preheat your oven to 350°F (175°C). Unroll one pie crust and place it in the bottom and up the sides of a 12-inch cast iron skillet. Use a fork to pierce the bottom and sides to prevent bubbling.

Place the skillet with the crust in the preheated oven and bake for 7 minutes. Remove and set aside. Increase the oven temperature to 400°F (200°C).

In a large bowl, mix together sugar, flour, and melted butter until crumbly. Gently toss the frozen blackberries in the mixture until evenly coated.

Pour the coated blackberries into the par-baked crust in the skillet. Evenly sprinkle 1/2 cup of water over the berries.

Unroll the second pie crust and place it over the blackberries, sealing the edges to the crust already in the skillet. Dot the top crust with small cubes of butter and sprinkle 3/4 cup of sugar over the top. Use a knife to cut small slits in the crust to vent steam.

Place the skillet in the oven and bake for 45 minutes, or until the filling is bubbling and the top crust is golden brown. If the edges darken too quickly, use a pie shield or aluminum foil to protect them.

Let the blackberry cobbler cool slightly before serving. Scoop generous portions into bowls and serve warm, optionally topped with vanilla ice cream.