Preheat oven to 350 degrees F. Line muffin tins with cupcake wrappers. Set aside.

In the bowl of a stand mixer fitted with a paddle attachment, add chocolate boxed cake mix. Mix on low speed to break up any clumps. Add buttermilk and vegetable oil and mix on medium speed until flour mixture is moistened.

Add eggs one at a time, making sure each egg is incorporated before adding the next. Once all the eggs have been combined, slowly stream in Dr. Pepper with the mixer running on low. Mix until combined.

Distribute batter among prepared muffin tin. Fill muffin tins 2/3 full for muffins to rise at same level as cupcake wrapper. Fill muffin tins 3/4 full for muffins to rise above cupcake wrapper. Bake for 18-22 minutes until toothpick inserted in center of cupcake comes out clean. Rotate muffin tin midway though baking to ensure even cooking. Let cupcakes rest in tin for 3 minutes before unmolding. Allow cupcakes to cool to room temperature on wire rack before frosting and decorating.

In the bowl of a mixer fitted with a paddle attachment, cream together butter and shortening until smooth.

In another bowl, sift together confectioners/powdered sugar and cocoa powder. Add half of mixture to mixing bowl with butter. Mix on medium speed until incorporated.

Slowly add Dr. Pepper with the mixer running on low. Add remaining half of sugar-cocoa mixture and mix on medium speed until combined. Transfer to pastry bag or zip-lock bag to pipe frosting onto cooled cupcakes. Garnish with maraschino cherry and chocolate jimmies for garnish.