Mummified cakes come alive with green jelly-bean eyes and gaping gummy-candy mouths. It's the perfect Halloween dessert for kids and adults alike.
Cupcakes: Preheat oven to 350 degrees. Line 18 standard muffin tins with paper liners. With an electric mixer on medium speed, beat flour, cocoa, sugar, baking soda, baking powder, and salt to combine. Reduce speed to low. Add eggs, buttermilk, oil, vanilla, and the water; beat until smooth, scraping down sides of bowl as needed.
Divide batter evenly among lined cups, filling each about two-thirds full. Bake, rotating tins halfway through, until a cake tester inserted in centers comes out clean, about 20 minutes. Transfer tins to wire racks to cool 10 minutes; turn out cupcakes onto racks and let cool completely. Cupcakes can be stored overnight at room temperature, or frozen up to 2 months.
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.
Spread each cupcake with a thin layer of buttercream. Add jelly beans dotted with melted chocolate for eyes and red gummy candy for mouth. Using a pastry bag fitted with a ribbon tip (such as #44), pipe crisscrossing bands of buttercream.