Preheat the oven to 180°C (Gas mark 4/350°F). Line a 12 hole muffin tin with 12 cupcake cases.
Place the butter in a mixing bowl and whisk together with the caster sugar until light and fluffy. Break the eggs into a bowl and lightly whisk with a fork. Add the eggs a little at a time to the butter and sugar mix, whisking in between.
Sift the flour, cocoa powder and baking powder on top, and using an electric whisk on a low speed, gently blend the ingredients together.
Add the milk and vanilla extract and blend again. Increase the mixer speed and whisk for a few more seconds until well mixed.
Divide the cake mixture between the cupcake cases. Bake for 20-25 minutes until risen and springy to the touch. Once the cakes are cooked, transfer to a wire rack to cool.
When ready to decorate, make the buttercream icing:
Whisk the butter with an electric whisk until light and fluffy. Sift in the icing sugar, a little at a time, until completely combined. Add the vanilla extract and whisk again. Add the milk in small splashes until the icing is at the correct consistency for piping.
Split the buttercream between two bowls, then add the orange and green gel food colourings and stir well until each colour is even.
Once the cakes are completely cool, spoon the green buttercream into a piping bag with a star piping nozzle. Pipe a swirl of green icing onto the top of half of the cupcakes.
Repeat with the orange buttercream, adding swirls of orange icing to the remaining cupcakes.
Add edible candy eyes to the buttercream swirls to turn your cupcakes into monsters.