Preheat the oven to 350°F. Spray a 13x9-inch baking dish with nonstick spray.
In a large microwave-safe bowl, microwave the chocolate and butter for 1 to 2 minutes, or until melted and smooth, stirring at 30-second intervals. Whisk in the sugar and eggs until well-blended. Stir in the flour Spread the batter into the prepared pan.
Bake for 25 to 30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Sprinkle the warm brownies with the toasted pecans and marshmallows.
PREPARE THE FROSTING:
In a large saucepan, cook the butter, milk and cocoa, stirring constantly, for 4 to 5 minutes or until the butter melts. Remove the pan from heat, and use an electric mixer to beat in the powdered sugar and vanilla at medium speed until smooth.
Pour the frosting over the pecans and marshmallows; spread to the edges. Let cool for 1 hour on a wire rack. Cut into squares. If they're too messy, you may with to serve them on a plate with a fork!