Prepare the Cake: Preheat oven to 325˚F. Grease a metal 13- x 9-inch baking dish with cooking spray. Place butter and chocolate in a microwavable bowl. Microwave on HIGH, stirring every 30 seconds until melted, about 1 minute 30 seconds total. Whisk together sugar, eggs, and melted chocolate mixture in a medium bowl. Combine flour, cocoa, baking powder, and salt in a separate bowl; stir into melted chocolate mixture. Fold in pecans and vanilla.
Spoon batter into prepared baking dish. Bake in preheated oven until a toothpick inserted in the center comes out with moist crumbs, 20 to 21 minutes. Remove cake from oven, and cover top with marshmallows in an even layer. Bake at 325°F until marshmallows are just soft, about 2 minutes.
While cake is baking, prepare the Chocolate Frosting: Heat butter, milk, and cocoa in a medium saucepan over medium until butter is melted, about 2 minutes. Increase heat to medium-high, and bring mixture to a boil. Boil 1 minute, and then remove from heat. Whisk in powdered sugar and vanilla. Immediately drizzle cake with Chocolate Frosting. Let Frosting harden about 2 hours before cutting cake into squares.