Microwave the chocolate and butter in a large microwave-safe bowl at HIGH 1 to 1 ½ 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 a greased 13- x 9- inch baking dish.
Bake at 350 F for 25 to 30 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Sprinkle the warm brownies with the toasted pecans and marshmallows.
Prepare the Chocolate Frosting:
Cook the butter, milk and cocoa over medium in a large saucepan, stirring constantly, 4 to 5 minutes or until the butter melts. Remove from the heat, and beat in the powdered sugar and vanilla with an electric mixer at medium speed until smooth. Pour over the pecans and marshmallows; spread to edges. Let cool 1 hour on a wire rack. Cut into squares.