Mix flour, sugar, and baking soda in large bowl and set aside.In a pot, melt 1 cup of butter, 1 cup of Coca-Cola, and 2 Tbsp. cocoa powder. Once the mixture has come to a boil, remove from heat and add 1 1/2 cups marshmallows and 1 tsp vanilla. Stir until marshmallows have melted.
To flour mixture, add 1/2 cup buttermilk, 2 eggs, and stir. Then slowly add in the cooked mixture. Once it is all well mixed, bake in 350º oven for about 30-35 minutes or until toothpick inserted in center comes out clean.
To make the icing, melt 1/2 cup butter with 1/4 cup Coca-Cola, 3 Tbsp cocoa powder, and 3 cups powdered sugar. Once it comes to a boil and everything is well dissolved, remove from heat and add 1 tsp vanilla. Pour over the top of warm cake.