I used a regular box of cake mix from Walmart. For decorating, a regular sized Nilla waffer for the "mouth area" and mini Nilla waffers that I cut in half for the ears. (You will go through a lot, as they tend to break) For the eyes I used mini marshmallows, that I just pressed in to the frosting. To add the small details for the eyes, nostrils and mouth I got a small tube of black icing gel.
These were very time consuming, but a ton of fun. Almost every monkey looked different. I stuck to three different facial expressions, but I am sure they would look even better with more!!