Whisk together melted butter and next 5 ingredients in a large bowl. Stir in flour and chopped pecans. Pour batter into a greased and floured 15- x 10-inch jellyroll pan.
Bake at 350° for 20 to 25 minutes or until a wooden pick inserted in center comes out clean. Remove from oven; top warm cake evenly with marshmallows. Return to oven, and bake 5 minutes. Drizzle Chocolate Frosting over warm cake. Cool completely.
Note: 2 (19.5-ounce) packages brownie mix, prepared according to package directions, may be substituted for first 7 ingredients. Stir in chopped pecans. Bake at 350° for 30 minutes. Proceed with marshmallows and frosting as directed.