Sift flour and granulated sugar together, stir in marshmallows and set aside. In a medium saucepan over medium heat, bring to a boil: butter, cocoa, and cola, stirring occasionally.
Pour into a 9 x 13 cake pan and bake at 350 degrees for 30 minutes. Or until cake tester comes out clean and cake bounces back when lightly pressed. Remove from oven and let cool on wire rack in cake pan for 30 minutes before adding frosting.
In large bowl, sift powdered sugar. Set aside. In a medium sauce pan, bring butter, cocoa, and cola to a boil, stirring frequently. When smooth, remove from heat and add to powdered sugar. Add vanilla extract, whisk until smooth. Pour frosting over warm cake, top with toasted pecans if desired.