Preheat the oven to 350°F.
Lightly spray a 9 x 13 baking dish with baking spray (Baker’s Joy or Wilton’s Bake-Ease)
Remove the cherries from the jar, reserving the liquid in the jar to be used in making the cherry frosting. Remove the stems of 15 of the cherries, slice in half, then slice the halves in half again. Keep the remainder of the cherries in the jar with the stems on to garnish the tops of the frosted pieces of cake.
Using a stand mixer or a handheld mixer and a large mixing bowl, add the dry cake mix and the cherry Dr Pepper. Mix on low until the dry cake mix and Dr Pepper are just combined.
Using a large spoon, stir in the sliced cherries.
Pour the batter into the prepared baking dish. Bake for 28 minutes, or until a knife or toothpick inserted comes out clean.
Set the cake aside and allow the cake to cool completely.
Using a stand mixer or a handheld mixer and a large mixing bowl on medium speed, cream together the softened butter and reserved cherry juice, about ¼ to ⅓ cup.
Reduce the mixer speed to low, and add the 3 cups of powdered sugar ½ cup at a time. This should be a firm frosting consistency, like canned frosting. If the frosting is thin, add up to ½ cup of additional powdered sugar. Be careful to not make the frosting so thick that it will not spread easily.
Spread the frosting evenly on top of the cake.
Next, gently score the size of the serving pieces of cake.
Place one of the reserved cherries with stems in the middle of each piece of cake. If the amount of stemmed cherries is less than the number of servings, remove the cherry stems, slice them in half, and garnish the tops of the pieces of cake.
To help the frosting set before cutting the individual pieces, place the cake in the refrigerator to chill for 30 minutes to 1 hour.
If using store-bought chocolate shavings, sprinkle the shavings evenly over the top of the frosting. Or using a vegetable peeler and a chocolate bar, gently scrape down the sides of the chocolate to make the shavings.