Preheat oven to 350°. Bake pecans in a single layer in a shallow pan 6 to 8 minutes or until lightly toasted and fragrant.
Microwave chocolate and butter in a large microwave-safe bowl at HIGH 1 to 1 1/2 minutes or until melted and smooth, stirring at 30-second intervals. Whisk in sugar and eggs until well blended. Stir in flour. Spread batter into a greased 13- x 9-inch pan.
Bake at 350° for 25 to 30 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Sprinkle warm brownies with toasted pecans and marshmallows.
Prepare Chocolate Frosting. Pour over pecans and marshmallows; spread to edges. Let cool 1 hour on a wire rack. Cut into squares.