In a large bowl, combine the first 7 ingredients, and beat the mixture on low speed for about 30 seconds.
Beat on medium speed for 2 minutes. Then fold in the marshmallows.
Pour into a greased 13x9 inch baking pan. Bake at 350° for about 35 to 40 minutes or until a toothpick inserted in the center comes out clean.
Cool on a wire rack for 15 minutes.
For the frosting while waiting for the cake to cool, combine butter and cocoa in a small saucepan.
Cook over low heat until the butter is melted. Stir in the cola until blended, Bring to a boil, stirring constantly.
Remove from the heat, stir in the confectioners' sugar until smooth. Fold in pecans.
Spread over the cake. Let it stand for 20 minutes before cutting the cake.