For crust: On a lightly floured surface, unroll 1 piecrust. Lightly brush with 2 teaspoons water. Top with remaining piecrust; roll into a 14-inch circle. Transfer to a 10-inch cast-iron skillet, pressing into bottom and up sides. Trim excess dough to ½ inch beyond edge of pan. Fold edges under, and crimp as desired. Prick dough all over with a fork. Top with a double layer of parchment paper, letting ends extend over edges of pan. Add pie weights.
Bake until edge of crust is lightly browned, about 14 minutes. Carefully remove paper and weights. In a small bowl, whisk together egg and remaining 2 teaspoons water. Brush edge of crust with egg wash. Bake until bottom of crust is golden brown, 12 to 14 minutes more. Let cool completely on a wire rack.
For filling: In a large saucepan, whisk together brown sugar, cornstarch, and salt; whisk in chocolate milk. Bring to a boil over medium heat, stirring constantly. Remove from heat. Place egg yolks in a bowl; whisk in ½ cup hot milk mixture. Whisk egg mixture into remaining hot milk mixture in saucepan. Cook until thickened and bubbly, about 2 minutes. Remove from heat. Add butter, 1 tablespoon at a time, stirring until butter is melted. Add chocolate and vanilla, stirring until chocolate is melted. Spoon filling into prepared crust. Let stand for 10 minutes. Cover surface of filling with plastic wrap. Refrigerate until cold and firm, about 6 hours.
For topping: In a large bowl, beat cream, confectioners’ sugar, cocoa, and vanilla with a mixer at high speed until stiff peaks form. Spread onto pie. Sprinkle with grated chocolate.
Recipe by Southern Cast Iron at https://southerncastiron.com/chocolate-cream-pie/