Heat oven to 350° F. Spray bottom only of 13x9-inch pan with cooking spray.
In medium bowl, stir brownie mix, oil, egg, cola and buttermilk until well blended. Spread into pan.
Bake 24 to 28 minutes or until toothpick inserted 2-inches from side comes out clean or almost clean. Cool completely, about 1 hour.
In medium microwavable bowl, microwave butter on HIGH 45 to 60 seconds, stirring once, until melted; stir in cocoa and cola. Gradually beat in powdered sugar.
Spread frosting over brownies; sprinkle with pecans. For bars, cut into 6 rows by 4 rows.
To toast pecans, preheat oven to 350° F. Spread pecans in a single layer in a baking pan. Bake about 5 to 7 minutes or until golden and toasted.
No buttermilk on hand? Combine 1 1/2 teaspoons vinegar with enough milk to equal 1/2 cup. Stir and let stand 5 minutes.