These ones look like they were made by a five year old. I am 27, and have no excuse. YMMV.
Step 1: Collect!
Baking powder (if your flour isn't self raising).
(This is actually adapted from a muffin recipe, but for some reason comes out tasting like scones. Savoury works quite well with the amount of chocolate you're going to be putting on these)
White chocolate (2 - 3 decent sized bars should be enough)
Green food colouring
Cooking oil (Can substitute milk - this is to loosen the chocolate and stop it binding if it dries out during melting).
White chocolate buttons.
Various mixing bowls, whisks, spoons, pans etc. as required.
A small muffin tin. I recommend the silicone type shown in this instructable, as it gives the closest to a mushroom shape I've found and they also pop out of it very easiy without any greasing.