For the sponge: Heat the oven to 160°C fan/gas 4 and line a 12-hole tin with paper cases. Add the butter and the sugar to a freestanding electric mixer and beat on medium speed until completely combined.
Turn the mixer up to full speed, then slowly add the egg and mix until completely combined.
In a small bowl, mix the cocoa powder, red food colouring and vanilla extract until it forms a dark paste.
Add to the butter mix and blitz in the blender until the batter is well combined. Turn the mixer down to a low speed and gradually add in half the buttermilk.
Beat until well mixed and then add half the flour and mix everything until well incorporated. Repeat until all the buttermilk and flour have been added. Turn up the speed to high an beat for a few minutes until the texture is smooth.
Add the mixture into the paper cases until two-thirds full and place in the oven to bake for 20 minutes until springy to the touch. Leave to cool in the tray before turning out onto the wire rack.
For the cream cheese frosting: beat the icing sugar and butter together in an electric mixer until the mixture is well combined.
Add the cream cheese and beat until well mixed. Continue to beat for another few minutes until the frosting is light. Place in the fridge to chill until needed.
When the cupcakes have cooled fully, pipe the frosting onto the cupcakes. Top with a red skull from a bag of The National Confectionery Company Spooky Treats.