Preheat your oven to 350°F (175°C). Line your cupcake pan with cupcake liners.
In a large mixing bowl, whisk together the sugar, flour, cocoa powder, baking soda, baking powder, and salt until well combined.
Add the egg, buttermilk (or sour milk), vegetable oil, and vanilla extract to the dry ingredients. Mix until just combined.
Carefully stir in the boiling water until the batter is smooth.
Pour the batter into the prepared cupcake liners, filling each about two-thirds full.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean. Let them cool in the pan for about 5 minutes before transferring them to a cooling rack.
Once the cupcakes are cool, frost them with your favorite buttercream frosting, saving some for the spiderweb decoration.
Using a piping bag, pipe a circle of frosting on each cupcake, then create concentric circles and drag from the center to the edges to create a spiderweb effect.