Preheat oven to 350 degrees Fahrenheit.

In a large mixing bowl, combine flour, granulated sugar, and baking soda.

Set the dry ingredients aside.

In a medium saucepan, combine cola, butter or margarine, and cocoa powder.

Bring the mixture to a boil over medium heat, stirring constantly.

Pour the hot cola mixture into the dry ingredients.

Stir until just combined.

Add eggs, buttermilk, and vanilla extract to the batter.

Stir until combined.

Gently fold in miniature marshmallows.

Pour the cake batter into a greased and floured 13x9x2-inch baking pan.

Bake in the preheated oven for about 30 minutes.

Check for doneness by inserting a wooden toothpick near the center of the cake.

If the toothpick comes out clean, the cake is done.

Transfer the cake in the pan to a wire rack.

Prepare the Cocoa Topping while the cake is cooling.

In a medium saucepan, combine butter or margarine, cocoa powder, and cola.

Bring the topping mixture to a boil.

Remove from heat and add powdered sugar and vanilla extract.

Mix with a wire whisk until smooth.

Stir in chopped nuts.

Immediately spread the Cocoa Topping evenly over the warm cake.

Allow the cake to cool completely on the rack before cutting into squares.

Serve and enjoy!