Whisk the flour and sugar together. Use a large mixing bowl. Make sure they are well combined. Set this dry mix aside.

Combine butter and cola in a pan. Add the cocoa powder as well. Bring the mix to a boil. Stir it constantly with a spoon.

Pour the hot liquid over flour. Stir the mixture very gently. Do not overmix the batter now. Keep it smooth and dark.

Add buttermilk and eggs to batter. Stir in the baking soda too. Add the vanilla and marshmallows now. The marshmallows will float on top.

Pour the batter into the pan. Bake for about twenty minutes. Use a toothpick to check doneness. It should come out clean.

Make the frosting while cake bakes. Boil butter and cola together. Stir in cocoa and powdered sugar. Add pecans for a crunch.

Pour warm frosting over warm cake. This is a very important step. It helps the frosting soak in. Let it cool before serving.