1. Preheat the oven to 350°F. Line a cupcake pan with Christmas cupcake liners.
2. In a large bowl, whisk together the cake mix, milk, eggs, oil, vanilla, cocoa powder, and dry pudding mix until smooth and thick. The batter will be richer and more velvety than a standard box mix.
3. Fill the liners about three quarters full to create tall, domed cupcakes.
4. Bake for 17 to 20 minutes, until the tops spring back lightly and a toothpick inserted in the center comes out with moist crumbs.
5. Let the cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before frosting.

Frosting

1. Beat the butter until light and creamy.
2. Gradually add the powdered sugar, mixing until smooth. Add the vanilla and peppermint extract if using, then beat in the heavy cream a little at a time until the frosting is light, fluffy, and easy to pipe.
3. Transfer the frosting to a piping bag fitted with a star tip. Pipe a swirl onto each cooled cupcake, then finish with glittery sugar and holiday icing decorations.