Feed everyone at Party Magic mushrooms and tell them the cake is in shape of a Fairy Toadstool....should work fine with minimum effort on your part.....
On a serious note - you could use a ready made chocolate or white sponge roll as the trunk, add ready to roll icing, decorate accordingly - a door, window etc. For the top, you could make a round sponge, shape to a semi circle, fit on top of log (laying down flat - wouldn't work standing up) and then spread with jam/butter cream and add ready to roll icing on the top - colour it accordingly.....