Preheat oven to 350F. Lightly spray a 9x13-inch baking pan with cooking spray and set it aside.
In a medium mixing bowl, combine flour and granulated sugar.
In a small saucepan, heat 1 cup butter, 3 tablespoons cocoa, and 1 cup cola soft drink to boiling and pour over the flour and sugar; mix thoroughly.
Add buttermilk, eggs, baking soda, vanilla, and marshmallows to the flour mixture and mix well.
Pour the cake batter into the prepared baking pan and bake for 40 minutes.
Immediately after the cake is removed from the oven to a cooling rack, prepare the frosting by combining the butter with cocoa, cola soft drink, and heat the mixture to boiling. Add in confectioners' sugar and mix well. Mix in chopped nuts and immediately spread the frosting over the hot cake.
Notes
Store the cake covered at room temperature for up to 3 to 5 days.