Grease and flour a 9 x 13 inch baking pan. (I used a glass pan) In a saucepan combine coca cola, oil, butter, and cocoa. Bring to a boil. In a separate bowl combine sugar, flour and salt. Pour the cola mixture over the flour mixture and stir together well.
Then add in the eggs, buttermilk, baking soda and vanilla and beat well. Gently fold in the mini marshmallows and pour into your prepared pan. Spread evenly in pan.
Bake at 350 degrees for 30 minutes.
Frosting
In a saucepan combine the butter, cocoa and milk. Stir until butter is melted and all mixed together. Pour into a mixing bowl and beat in vanilla and powdered sugar.