Make the 2 packages of brownie mix according to directions.
Stir in chopped pecans.
Pour batter into a greased and floured 15- x 10-inch jellyroll pan.
Bake at 350° for 20 to 25 minutes until mostly done.
Remove from oven; top with marshmallows while still warm.
Return to oven, and bake another 5 minutes – just long enough to start “browning” the marshmallows. Remove from oven.
Microwave frosting for 45 seconds or so until pretty melted. Drizzle chocolate frosting over warm cake. Cool completely.