They are basic chocolate cupcakes, which were filled with 'slime green' cream cheesiness and topped with buttercream skulls. Peanut M&Ms and chocolate Jimmies provide the decorative details.
Lessons: Plain M&Ms can be used for eyes if you want a more uniform look -- since some of those peanuts are huge. Or, you can combine plain and peanut for a kooky appearance without worrying that the eyes will overwhelm the skull.
Also, mini-chocolate chips would make a better nose (hole) since they're triangular in shape naturally.
Jimmies make cute "teeth," but I would buy a large container next time for more options. Also, a pair of kitchen tweezers makes it easier to select and place the candies and embellishments. I didn't have a pair then, but I've since invested in a pair--they're larger than typical tweezers and the tips are off-set.
If/when I make these again, I also might put a light layer of chocolate frosting or ganache down first, so that the cupcake top will be covered to the edges.
