Preheat oven to 400°. Position oven rack in the bottom third of oven.
Let All-Butter Piecrust stand at room temperature until slightly softened, 10 to 15 minutes. On a lightly floured surface, roll dough into a 16×13-inch rectangle (about ⅛ inch thick). Press into bottom and up sides of a 13×9-inch baking sheet, letting excess extend over sides of pan. Roll crust up towards center; crimp as desired. Freeze until firm, about 15 minutes. Top with a piece of parchment paper, letting ends extend over edges of pan. Fill with ceramic pie weights.
Bake for 15 minutes; remove parchment and weights. Bake until crust is dry to the touch (but not fully baked), about 5 minutes more. Let cool on a wire rack.
Reduce oven temperature to 325°.
In a large microwave-safe bowl, combine chocolate and butter; heat on high in 30-second intervals, stirring between each, until melted and smooth (about 1½ minutes total). Whisk in sugar, eggs, and vanilla until combined.
In a medium bowl, whisk together flour, hot chocolate mix, baking powder, and salt. Stir flour mixture into chocolate mixture. Spoon into prepared crust. Smooth with an offset spatula.
Bake until a wooden pick inserted in center comes out clean, 35 to 40 minutes, loosely covering with foil to prevent excess browning, if necessary. Let cool completely on a wire rack.
Preheat broiler.
Sprinkle pie with marshmallows. Broil on middle rack of oven until lightly toasted, about 1½ minutes. Garnish with chocolate, if desired.
All-Butter Piecrust: Freeze butter for 15 minutes.
In the work bowl of a food processor, place flour, sugar, and salt; pulse until combined. Add cold butter; pulse until mixture is crumbly and butter is in pea-size pieces. With processor running, pour 5 tablespoons ice water through food chute in a slow, steady stream just until dough comes together. (Mixture may appear crumbly. It should be moist and hold together when pinched.) Add additional 1 to 2 tablespoons ice water only if needed. Shape dough into a rough 5×4½-inch rectangle. Wrap tightly in plastic wrap and refrigerate for at least 1 hour or up to 2 days.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/hot-chocolate-slab-pie/