Preheat the oven to 350°F (175°C). Lightly coat a 9×13-inch baking dish with nonstick baking spray. Set aside.

Remove 15 maraschino cherries from the jar, ensuring stems are removed. Slice each cherry in half, then slice the halves again to quarter them. Reserve the remaining cherries (with stems) in the jar for garnishing after baking.

In a large mixing bowl, combine the dry cake mix with the cherry Dr Pepper. Using a hand mixer or stand mixer on low speed, blend until just combined. Gently fold in the chopped cherries with a spatula or large spoon.

Pour the prepared batter into the greased baking dish. Bake for 28–30 minutes, or until a toothpick inserted in the center of the cake comes out clean. Once baked, set aside and allow the cake to cool completely in the pan.

In a separate mixing bowl, cream the softened butter with ¼ to ⅓ cup of the reserved cherry juice using a hand mixer or stand mixer on medium speed. Once well blended, reduce the speed to low and gradually add 3 cups of powdered sugar, ½ cup at a time. Mix until the frosting reaches a firm, spreadable consistency. If the frosting appears too thin, incorporate up to ½ cup of additional powdered sugar.

Evenly spread the cherry frosting over the cooled cake using an offset spatula or spoon. Lightly score the frosting to indicate serving portions. Place one maraschino cherry (with stem) in the center of each slice. If fewer than 12 cherries remain, remove the stems and halve the cherries to ensure even garnish distribution.

Refrigerate the frosted cake for 30 to 60 minutes to allow the frosting to set before slicing. If desired, garnish the top with chocolate shavings. For homemade shavings, gently scrape the side of a chocolate bar using a vegetable peeler.