Preheat oven to 350 degrees. Combine flour and sugar in large bowl. In small saucepan, heat 1 cup butter, 3 tablespoons cocoa and 1 cup Coca-Cola. Bring to simmer and then pour over flour mixture. Stir well. Add buttermilk, eggs, baking soda and vanilla. Mix. Add marshmallows. Pour into 9x13 pan for 35 minutes.
To make the frosting, in a saucepan combine 1/2 cup butter, 6 tablespoons Coca-Cola, and 3 tablespoons cocoa. Bring to a boil. Pour over bowl with powdered sugar. Beat well. Spread evenly over warm cake. Let cool. Serve.
Recipe Courtesy of Dotson's Restaurant