put rack in center of oven and preheat to 350*F. Lightly spray a 9x13 pan with vegetable oil. Set aside.
Place first 7 cake ingredients in your mixer bowl and mix till batter looks well combined. Stir in mini marshmallows. Pour into prepared pan and smooth top with spatula.
Bake cake till the top springs back when touched and sides are just beginning to pull away from pan. 40-42 minutes. Remove pan to wire rack to cool for 15 min.
Meanwhile, prepare the frosting. Put butter in a med. saucepan. As the butter melts add cocoa and cola. Allow mixture to come to a boil stirring constantly. Remove from heat. Stir in Sugar until smooth and thick. Fold in pecans.
Pour the frosting over the top of the cake, spreading with a rubber spatula so it reaches the edges of the cake. Cool 20 minutes before cutting.