-- Combine
Coca-Cola and buttermilk; set aside.
-- Beat
butter at low speed with an electric mixer until creamy.
-- Gradually
add sugar; beat until blended.
-- Add
egg and vanilla; beat at low speed until blended.
-- Combine
flour, cocoa, and soda.
-- Add
to butter mixture alternately with cola mixture; begin and end with
flour mixture.
-- Beat
at low speed just until blended.
-- Stir
in marshmallows. Pour batter into a greased and floured 13- x 9-inch
pan.
-- Bake
at 350° for 30 to 35 minutes.
-- Remove
from oven; cool 10 minutes. Pour Coca-Cola
Frosting* over warm cake; garnish, if desired.