I set out to make cookies with my toddler and I think I found a decent approach to it. 
The first thing I did was wait for him to go down for a nap...that way I could get everything setup and ready for him when he woke up from it! I used paper bowls to hold the ingredients he could help me with that way there would be no broken dishes. I also got the mixer all setup as well.
Once he woke up from his nap I put all of the wet ingredients and sugars in the mixer bowl. I stood next to him before I turned it on though because I wasn't sure how he would react to it and he was after all, standing on a stool. Well, good thing I was thinking because it did scare him! I made sure to warn him and be there when we needed to use it.
He poured (most) of the dry ingredients in the bowl though some made it onto the floor too. When it came time to put the butterscotch chips in he reached his hand in, grabbed a handful, and then shoved them in his mouth. After that I had to take over and add the 
bad myself or otherwise they would not have made it in!
Finally, I chose to make a cookie bar instead of cookies. This made it a much easier and faster experience and gave me extra time to clean up the messes. Overall I think it was a good experience for both of us :-)