Preheat the oven to 325°F. Line 2 standard muffin pans with 24 cupcake liners.
In a large mixing bowl, whisk together the cake mix and instant pudding mix. Stir in the chocolate chips.
Add the eggs, vegetable oil, water, and sour cream. Mix until the batter is smooth and well combined. Do not overmix.
Fill each cupcake liner about ¾ full.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cupcakes in the pans for 5 minutes, then transfer them to a wire rack to cool completely before frosting.
For the Frosting:
In a large mixing bowl, beat the softened butter for 2-3 minutes until light and creamy.
Add the powdered sugar, cocoa powder, heavy whipping cream, 1 teaspoon vanilla extract, and salt.
Mix on low speed until combined, then increase the speed to high and beat for 2-3 minutes until light and fluffy.
If needed, add additional heavy cream, 1 tablespoon at a time, until the frosting reaches a spreadable consistency.
To Assemble the Cupcakes:
Spread an even layer of frosting over each cooled cupcake, reserving a small amount of frosting for attaching the decorations.
Press 1 vanilla wafer near the bottom of each cupcake to form the reindeer's muzzle.
Insert 2 pretzel twists above the wafer to create the antlers.
Place the reserved frosting into a piping bag (or zip-top bag with a small corner snipped off). Pipe a small dot of frosting onto the backs of the candy eyes and M&M's.
Attach 2 candy eyes above the vanilla wafer and 1 red M&M on the wafer for Rudolph's nose.
Serve immediately or store until ready to enjoy.
Notes
Store the decorated cupcakes in an airtight container in the refrigerator for up to 4 days. Bring to room temperature before serving.
If making ahead, bake the cupcakes one day in advance and decorate the day you plan to serve them.
For the best texture, use block-style cream cheese and full-fat sour cream if substituting frosting recipes.