I made the WW one...or used it as a base too...I bought a few packets of the home brand butter cake mix, and used the microwave instructions. All I had to do was use 2 slightly different shaped microwave safe bowls (which I already had) and then ice and put together. I ended up using some skewers through the cake to hold the 2 parts together, and cut a plastic plate to fit on the underside of the cake to be the top of the mushroom, just for a bit of extra support.

Good luck! And remember to have fun!

Oh, and Woolworths had some little sugar icing fairies which I used to put around the toadstool. And I bought some of the premade white fondant icing (around $5 for a small box at woolies) to make the little white circles to go on my toadstool. If I can find a photo I'll put it up tonight.