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Heat oven to 180C/160C fan/gas 4. Line two 12-hole cupcake tins with cases. Beat the butter and sugar until the mixture is creamy. Beat in the chocolate and the eggs until combined, then stir in the flour and baking powder, cocoa powder, milk and food colouring, if using. Spoon the mixture evenly between the cupcake cases, levelling the tops.
Bake for 20 mins or until the cakes are risen and springy to the touch. Cool for 5 mins in the tin, then lift out onto a wire rack to cool completely.
To make the buttercream, beat the butter until soft, then stir in the icing sugar a little at a time. Beat in the vanilla and some black food colouring, then transfer to a piping bag fitted with a plain nozzle.
For the cat face, pipe the black buttercream in an even swirl onto the cupcake and smooth with a palette knife. Pipe two ears by making a blob for each and pulling the icing bag upwards to a point. Add two eyes on each with Smarties and use the black icing pen to paint a pupil onto each. Add a triangle nose made of pink fondant icing and create whiskers with sweets.
For the monster faces, cut circles of green and blue fondant to fit the tops of the cupcake and fix them in place with a little buttercream. Add eyes, noses and mouths made of sweets or use icing eyes (see tip, below). For skeletons, cut out shapes with white fondant and fix in place with the buttercream.