Preheat oven to 350 degrees
Line a 9 x 13 cake pan with parchment paper, set aside
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
Remove from heat and let cool for 5 minutes. Pour over dry mixture, whisk until combined
Whisk in buttermilk, baking soda, beaten eggs, and vanilla extract
Pour into 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