Use a knife to coat the whole cake with a thin layer of buttercream, it doesn't have to be perfect, just coated enough that the icing with grip it.
Now colour some ready roll icing with black gel food colouring until it is a good dark black colour and is really mixed in. Put plenty of icing sugar on your work surface and then roll out the black icing into a disc big enough to cover your little cupcake. Smooth the disk of icing over the cupcake and keep rubbing your hands over it until the extra icing sugar and the wrinkles and folds have been smoothed out of it. Trim the extra icing away from the top of the cake so that you have one face open. Next roll a sausage of black icing about 1/2cm thick and then fix it around the open face of the cake to make a lip for the cauldron. It should stick itself on if you are working quickly, if not you may need a little confectioners glue or a little water to help it stick.
Step 3: Adding the bubbles
Using a piping bag and wilton icing round tip number 12, pipe little disks of icing over the top of the cake to look like bubbles. I'm afraid this was my first attempt at icing with a piping bag so forgive the uneven bubble sizes! Next I used some tiny bits of white readyroll icing and made them into little balls then stuck a tiny ball of black in the middle to make them look like eye balls and threw them on top of the bubbles. I then used a little orange readyroll icing and rolled it into tiny sausages to look like fingers and threw them in the cauldron too. At the side I added a little stick of black readyroll icing to look like the big mixing spoon.