My last iron did that - I was very surprised because I do not leave water in it, I would always empty it and left it out to dry. And if I used starch it was suddenly burning the starch. It always washed out by just putting it back into the washing machine with detergent. Hopefully the Dawn will do the trick -- it seems to work when nothing else.
I got a new iron and it does not get any water. (I tried it once because I do like using steam but it just not doing what I wanted, so now I use a dry iron and spray water and/or starch when needed. (I also keep a used dryer sheet on my ironing board to run the iron over every few minutes to avoid any starch build up if I am using starch.)
By any chance does your iron have a teflon coating on the plate? My other one did and I was wondering if that was the problem with scorching.....