I did use paper towels/kitchen paper to smooth the buttercream down as the internet advises but my paper towel had quite a bumpy pattern AND it was the last sheet on the roll so had some creases on it so my finish was not quite perfect. Alas.
I bought black ready-made fondant icing from Tesco which tastes pretty decent. I printed the silhouette of Michael Jackson from a Google image search and put it over the rolled out icing, pricked all around it with a pin and then cut it out carefully before placing on top of the cake. I think it worked quite well and the birthday boy was pleased. My next challenge may well be making my own marshmallow fondant.
