Preheat the oven to 350°F.

Lightly spray a 9x13 baking dish with baking spray.

Remove the cherries from the jar, reserving the liquid for the frosting. Remove the stems from 15 cherries. Slice the cherries in half, then slice the halves again. Keep the remaining cherries with stems for garnish.

In a stand mixer or large bowl, combine the dry cake mix and cherry Dr. Pepper. Mix on low until just combined.

Stir in the sliced cherries.

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

In a stand mixer or large bowl, cream together softened butter and reserved cherry juice.

Reduce mixer speed to low. Gradually add powdered sugar, ½ cup at a time, until the frosting reaches a firm but spreadable consistency, similar to canned frosting. Add up to an additional 0.5 cup of powdered sugar if needed.

Spread the frosting evenly over the cooled cake.

Gently score the cake into serving portions.

Place one reserved cherry with a stem in the middle of each serving. If short on stemmed cherries, garnish with halved cherries.

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

Sprinkle chocolate shavings evenly over the top of the frosting. If using a chocolate bar, create shavings using a vegetable peeler.