Preheat the oven to 180 degrees Celsius (350 degrees Fahrenheit, 160 degrees fan oven).
To make the sponges, add the milk and butter to a pan set over a low heat, and leave them to heat up for the butter to melt.
Remove from the heat and set aside.
In a large bowl, use a hand mixer to cream together the egg and sugar until you get a smooth pale yellow paste.
Pour in the milk mixture.
Sift in the flour, cocoa powder and baking powder, add the vanilla extract and use a spatula to combine all the ingredients into a smooth batter.
Line a 6-hole muffin tin with paper cases, and divide the batter evenly.
Bake for 20 minutes or until a skewer inserted in the middle comes out clean.
Remove the cupcakes from the oven and leave them to cool down completely.
When the cupcakes are cool, make the buttercream.
Sift the icing sugar and black cocoa powder, and add them to a food processor together with the vanilla extract, butter and milk.
Blitz to get a smooth buttercream.
Use a piping bag fitted with a large star nozzle to decorate the cupcakes, then sprinkle some Halloween sprinkles over.