Preheat the oven to 350º Fahrenheit. Spray a 9x13 baking pan with cooking spray.
Whisk together flour and sugar in large mixing bowl.
In a small saucepan, heat 1 cup butter, cocoa, and 1 cup cola soft drink to boiling.
Pour over the flour and sugar and use a hand mixer to mix well.
Add buttermilk, eggs, baking soda, vanilla, and marshmallows and mix well.
Pour the batter in the prepared pan.
Bake for 40 minutes or until the cake tests done.
When the cake has about 10 minutes left of baking, make the frosting.
Make the Frosting:
In a small saucepan, melt butter, cocoa powder and cola. Heat over low heat just until the butter is melted.
Turn up the stovetop temperature slightly until the mixture is just barely boiling. Stir often.
Take off the heat right at boiling.
Place the powdered sugar in large bowl.
Pour the hot butter mixture over the confectioners' sugar. Use a hand mixer to mix well, scraping the sides of the bowl as needed.
Spread the frosting over the cake while it is still hot.
This cake is delicious served warm with a scoop of ice cream.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**