Preheat the oven to 350°F. Place cupcake liners into muffin cups.
Combine buttermilk, oil, and eggs in the bowl of your mixer. Add cake mix and cocoa powder to wet ingredients and mix together until combined.
Use an ice cream scoop to fill cupcake liners 2/3 full with batter. Bake for 15-17 minutes and remove from oven to cool.
Prepare the Buttercream Frosting
Beat the butter with an electric hand mixer or in the bowl of a stand mixer until light and fluffy; add salt and powdered sugar on low speed.
Add vanilla and heavy cream and mix till combined. Turn mixer to medium-high speed and beat buttercream for five minutes. If buttercream is too thick, add heavy cream 1 tsp at a time until desired consistency is reach.
Build the Cupcakes
Place the frosting in a 16oz frosting bag fixed with a Wilton 1M star tip. Pipe the frosting evenly over cupcakes.
Cut out Rudolph and Friends cupcake toppers and place one directly on top of each cupcake. Add sprinkles as desired.