Taken with my phone...so not a great pic.

 

This recipe is only very slightly adapted from this one found at CopyKat Recipes.  The only thing I changed was the type of soda used in the recipe, which you can change to whatever dark soda tickles your fancy.

1 Stick of Butter
1 C. Dr. Pepper
1 Square of Semi Sweet Baking Chocolate
3/4 C. Miniature Marshmallows
1/2 C. Shortening
1/2 C. Vegetable Oil
2 C. Sugar
2 Eggs
3/4 C. Buttermilk
2 tsp. Vanilla
1/2 tsp. Salt
3/4 C. Cocoa (I used Hershey’s Special Dark cocoa…it gives the cake more richness and body.)
2 1/3 C. Flour
1 tsp. Baking Soda
1 1/2 tsp. Baking Powder

Place in a sauce pan butter, Dr. Pepper, semi sweet baking square and bring to a boil. Remove from heat and add marshmallows and stir until all is blended and the marshmallows disappear. Set aside to cool to room temperature.
In mixing bowl blend the shortening, oil, vanilla and sugar until fluffy. Add eggs and buttermilk and beat until smooth. Mix flour with baking soda, cocoa, baking powder and salt. Add half of flour mixture to shortening mixture and beat. Scrape bowl time to time to clean the sides so ingredients are blended in batter. Add cooled coca cola mixture and beat. Add remainder of flour mixture and beat until completely smooth, about 3 – 4 minutes. Batter will be thick.
Place batter into a 13″ x 9″ x ” 2 oiled and floured baking pan.
Set pan on top shelf in a preheated 350 degrees oven for 35 – 40 minutes. Check for completed baking time with a toothpick.

Frosting
1 Stick of Butter
1/2 C. Cocoa
1/4 C. Dr. Pepper (plus 2-3 Tbsp. if needed)
3 C Powdered Sugar
1 tsp. Vanilla
1/4 C Hershey’s Chocolate Syrup

Cream butter, add cocoa, vanilla, Hershey’s Syrup, and Dr. Pepper, beat until smooth. Add powdered sugar a little at a time and beat constantly. Scrape the sides of bowl time to time to get all ingredients into the frosting. If the frosting seams a little dry add a little more coca cola and beat well.