Combine flour and white sugar in mixing bowl. Heat 1 cup butter, 3 Tbls cocoa, and 1 cup coca-cola to boiling and pour over the flour and sugar. Mix thoroughly.
Add buttermilk, eggs, baking soda, vanilla, and marshmallows and mix well.
Bake in a prepared 9x13 inch pan for 40 minutes at 350 degrees. Frost while hot.
COCA-COLA FROSTING: In saucepan, combine 1/2 cup butter, 2 Tbls cocoa and 6 Tbls cola in a sauce pan. Bring to a boil. Pour over 4 cups confectioner's sugar and mix well. Mix in chopped pecans and spread over hot cake. Frosting should be spreadable. If it is too thick, add a few drops of cola.