dark (black or dark brown) cupcake liners; a disposable pastry bag; a 1-inch square cutter; a 2-inch teardrop cutter
For the cupcakes: Preheat the oven to 350 degrees F. Line two cupcake pans with dark (black or dark brown) cupcake liners.
Add the butter and granulated sugar to the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until light and fluffy, 3 to 4 minutes. (Do not skimp on the creaming time!) Add the sour cream, orange zest and vanilla and beat until well combined. Add the egg whites in two batches, mixing until well combined after each addition. Scrape down the sides and bottom of the bowl as needed to be sure all the ingredients are well incorporated.
Combine the flour, baking powder and salt in a separate large bowl and whisk together. Combine the milk and orange juice in a measuring cup.
Add half the dry ingredients to the batter and mix until well combined. Add half the milk mixture and mix until well combined. Add the remaining dry ingredients and mix until well combined. Scrape down the sides and bottom of the bowl as needed to be sure all the ingredients are well incorporated. Add the last of the milk mixture and mix to combine, being sure not to overmix.
Use an ice cream scoop to fill the cupcake liners three-quarters of the way to the top. Bake until a toothpick inserted in the center of a cupcake comes out with a few crumbs sticking to it, 15 to 17 minutes. Allow to cool for 2 to 3 minutes, then remove to a cooling rack to finish cooling completely.
For the rum marshmallow filling: Add the marshmallow creme to a medium bowl. Stir in the rum until well combined. Transfer the filling to a disposable pastry bag.
For the frosting: Put the butter in the bowl of a stand mixer fitted with a paddle attachment. Beat on high speed until light and fluffy. Add the vanilla and salt and beat until combined. Add the cocoa powder and beat on low speed until combined. Add half the powdered sugar and half the milk. Beat on low speed until the ingredients are moistened, then turn the mixer to high and beat until well incorporated. Add the remaining powdered sugar and another 2 tablespoons milk; mix on low again until the ingredients are moistened, then turn the mixer to high and beat until fluffy. (This is a thick icing; if you prefer a thinner consistency, add more milk one tablespoon at a time until you reach your desired thickness.)
Use a paring knife to cut a small plug out of the center of the top of each cupcake, saving the cake plugs. Cut the tip from the pastry bag with the filling and fill the center of each cupcake with a small amount of marshmallow filling. Trim the bottoms of each cake plug and use to seal the holes in each cupcake.
To decorate: Frost each cupcake with the frosting, using an offset spatula to smooth the tops. Arrange the cupcakes next to each other with their edges touching in a 4-by-6 grid on a large cake board or platter.
Line a baking sheet with parchment paper. Roll out the green gum paste to about 1/8-inch thick and cut twenty-four 1-inch squares. Place on the lined baking sheet and let dry 15 minutes. Roll out the blue gum paste and cut four 2-inch teardrops; place on the lined baking sheet and let dry for 15 minutes. Repeat with each of the three remaining colors (pink, purple, red) to make a total of 12 more teardrops.
Brush the top of each of the blue teardrops with a small amount of the alcohol, then sprinkle with blue edible glitter. Repeat with each of the colored teardrops, sprinkling each with its coordinating colored glitter. Use a small amount of alcohol to moisten the top half of a green square, then position a teardrop on top, pointy-side out, to look like a small lightbulb on a string of lights. Repeat with the remaining teardrops and squares. Let dry for a few minutes.
Use an offset spatula to place a lightbulb on top of each cupcake, varying the colors and the direction they are pointing throughout the grid of cupcakes. Thread the licorice laces around, looping them and attaching them to lightbulb bases occasionally, to mimic a tangle of holiday lights.