Preheat your oven to 350°F (175°C). Line a cupcake pan with cupcake liners.
In a large mixing bowl, whisk together the sugar, flour, cocoa powder, baking soda, baking powder, and salt.
Add the egg, buttermilk, vegetable oil, and vanilla extract to the dry mixture. Mix until just combined.
Slowly stir in the boiling water until well combined.
Spoon the batter into the prepared cupcake liners, filling each about two-thirds full. Bake for 18-20 minutes.
Let the cupcakes cool in the pan for about 5 minutes, then transfer to a cooling rack.
Prepare your favorite buttercream frosting while the cupcakes cool.
Once cool, pipe a spiral of frosting to create a spiderweb effect. Add a decorative spider if desired.