Preheat oven to 350 degrees F. Grease 13x9x2-inch baking pan. In large bowl, stir together flour and granulated sugar. Set aside.
In medium saucepan, combine butter, cola, and cocoa powder. Cook and stir over medium heat until butter melts. Pour over flour mixture. Add eggs, buttermilk, baking soda, and vanilla; mix well. Stir in marshmallows. Pour into prepared baking pan.
Bake for 30 to 35 minutes or until a wooden toothpick inserted near center comes out clean. Cool cake in pan on wire rack.
Spread Cocoa-Cola Frosting over cake. Sprinkle with pecans and marshmallows, if desired.
In medium bowl, beat butter with electric mixer on medium to high speed for 30 seconds. Slowly add 2 cups powdered sugar and cocoa powder, beating until combined. Add cola, beating until combined. Gradually beat in 1-3/4 cups powdered sugar until combined. If necessary, beat in enough additional cola, 1 teaspoon at a time, to make frosting easy to spread.