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.
Note: Don’t make the frosting ahead–you need to pour it over the cake shortly after baking.
For the coca-Cola frosting:
Bring first 3 ingredients to a boil in a large saucepan over medium heat, stirring until butter melts. Remove from heat; whisk in sugar and vanilla.