I've been collecting recipes since I was in junior high school back in the '60s. I don't remember how old this recipe is, but I cut it out of a Southern Living magazine years ago. My friends at church love this cake and I don't ever have any leftovers! Hope you like it too! Double the recipe and it fits in a full size restaurant pan and feeds 32.
Beat butter on low speed with electric mixer until creamy. Gradually add sugar, beat until blended. Add egg and beat after each addition. Add vanilla and blend well.
Combine flour, cocoa, and soda. Add to butter mixture alternately with the cola mixture; begin and end with the flour mixture. Beat on low speed just until blended.
Stir in marshmallows. Pour into a prepared 9X13 pan. Bake at 350 degrees for 30-35 min. Remove from the oven; cool 10 min. Then pour frosting over warm cake.
For FROSTING: Bring the 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. Beat until smooth. Add chopped pecans. Then pour over warm cake. Frosting will set up and become firm when it cools.