When cooked and cooled you can decorate with the frosting.
For the frosting: combine confectioners' sugar, melted light butter, vanilla and enough water to achieve frosting consistency, starting with 1 tbsp and adding more if needed.
Place frosting into a piping bag, cut the tip off and pipe onto cupcakes. (no need for a fancy tip unless of course you want to use one!)
While frosting is still wet, press 2 m&m's (letter side down) of the same color on each cupcake to create the eyes.
*Note: Some "light" butters have yellow coloring added which will effect the color of the icing, Land O Lakes Light is a good option. Also, dark vanilla will effect the color, use clear to keep it white.