Preheat oven to 350°F (175°C).

Lightly spray a 9x13 inch baking dish with baking spray.

Drain the maraschino cherries, reserving the liquid for the frosting. Remove the stems from 15 of the cherries. Slice these cherries in half, then slice the halves again. Keep the remaining cherries with stems on for garnish.

In a large bowl, combine the dry cake mix and cherry Dr. Pepper. Mix on low speed until just combined. Do not add water, milk, eggs, or oil.

Stir in the sliced cherries with a large spoon.

Pour the batter into the prepared baking dish. Bake for 28 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool completely.

For the frosting: In a stand mixer or with a handheld mixer, cream together the softened butter and reserved cherry juice on medium speed until smooth. Ensure the butter is softened, not melted.

Reduce mixer speed to low. Gradually add powdered sugar, ½ cup at a time, until a firm frosting consistency is reached, similar to canned frosting. If too thin, add up to an additional ½ cup of powdered sugar. Be careful not to make it too thick to spread easily.

Spread the frosting evenly over the cooled cake.

Gently score the cake into serving-sized pieces.

Place one reserved cherry with a stem in the center of each piece of cake. If you have fewer stemmed cherries than servings, remove the stems from the remaining cherries and slice them in half to garnish the cake pieces.

Chill the cake in the refrigerator for 30 minutes to 1 hour to help the frosting set before cutting.

Sprinkle chocolate shavings evenly over the top of the frosting. If using a chocolate bar, use a vegetable peeler to gently scrape shavings from the sides.