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 baking 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 13x9 inch pan. Bake at 350 for 30 to 35 minutes. Remove from oven; cool 10 minutes. Pour Coca-Cola Frosting over warm cake. Garnish with pecans, if desired.
Note: Don't make the frosting ahead - you need to pour it over the cake shortly after baking.