In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. In another bowl, whisk together the buttermilk, oil, egg, and vanilla. Gradually mix the wet ingredients into the dry, then stir in the boiling water until smooth.

Preheat the oven to 350°F (175°C). Line a cupcake pan with liners. Fill each liner about two-thirds full with batter, then bake for 20-25 minutes or until a toothpick comes out clean. Let cool for 5 minutes before transferring to a wire rack.

To make the frosting, beat the softened butter until creamy. Gradually add the powdered sugar and milk, mixing until fluffy. Add food coloring to achieve the desired hue.

Once the cupcakes are cool, pipe the frosting onto each cupcake to create a brain-like appearance. Drizzle red decorating gel over the frosting for a spooky effect.