Mini Chocolate Cupcakes: Preheat oven to 350 degrees. Line mini muffin tins with paper liners. Sift cocoa powder, flour, sugar, baking soda, baking powder, and salt into a large bowl. Add eggs, warm water, buttermilk, oil, and vanilla, and mix until smooth.

Divide batter among muffin cups, filling each 2/3 full. Bake until tops spring back when touched, 8 to 10 minutes. Transfer cupcakes to wire racks, and let cool. Cupcakes will keep, covered, for up to 3 days.

Swiss Meringue Buttercream: Combine egg whites, sugar, and salt in the heatproof bowl of a standing mixer set over (not in) a pan of simmering water. Whisk constantly by hand until mixture is warm to the touch and sugar has dissolved.

Attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, whisk until the mixture is fluffy, glossy, and completely cool, about 10 minutes.

With mixer on medium-low speed, add the butter a few tablespoons at a time, mixing well after each addition. Whisk in vanilla. Switch to the paddle attachment, and beat on low speed until air bubbles are eliminated and the frosting is completely smooth. Keep buttercream at room temperature if using the same day, or refrigerate in an airtight container up to 3 days. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again.

Frost mini cupcakes with buttercream, then cover tops with sanding sugar. For each spider, use 2 cinnamon candies for eyes. For legs, attach 8 pieces of licorice. For fangs, cut out tiny cone shapes from a marshmallow and attach.