For the cupcakes: Preheat the oven to 350 degrees F. Line two 12-cup muffin tins with black paper liners.
Whisk together the cake flour, cocoa powder, granulated sugar, baking powder, baking soda and salt in the bowl of a stand mixer. Whisk together the buttermilk, coffee, oil, eggs and vanilla in a separate bowl.
Pour half of the wet ingredients into the dry ingredients and beat on low speed just until combined. Scrape down the bowl then add the remaining wet ingredients and beat to combine. Scrape down the bowl and paddle. Increase the speed to medium and beat for a full minute.
Fill the prepared cupcake tins 2/3 of the way full. Bake until a toothpick inserted in the center of each comes out clean and the top springs back when lightly pressed, 15 to 18 minutes.
For the frosting: Microwave the chocolate in a medium microwave-safe bowl, stirring occasionally, until smooth. Cool to room temperature. Meanwhile, beat the butter and salt in a large bowl with an electric mixer on medium-high speed until fluffy, about 2 minutes. Reduce the speed to low, then gradually add the confectioners’ sugar until combined. Increase the speed to medium and beat, scrapping down the sides of the bowl with a rubber spatula, until fluffy, about 2 minutes. Add the cocoa powder, milk and vanilla, then beat until smooth. Add the room temperature melted chocolate, then increase the speed to medium-high and beat until very light and fluffy, 2 to 3 minutes more.
Transfer the frosting to a large pastry bag fitted with a large round tip. Decoratively pipe the frosting onto the cupcakes in a circular, upward motion. Refrigerate until slightly firm, about 10 minutes.
For the marshmallow web: beat the marshmallows in a large bowl with a mixer fitted with the whisk attachment on low speed until the marshmallows break up, about 1 minute. Increase the speed to medium high and beat until glossy and smooth. Add the confectioners’ sugar, and beat again until very thick and smooth, about 1 more minute. Using two forks, pull out about 1 tablespoon of marshmallow at a time, stretch into thin strands and drape over the top and around each cupcake to create a web.
Stick one gummy spider candy on each, then let the marshmallow set, about 15 minutes.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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