Wendy - don't know the technical explanation for it, but especially with cookies, there can be a big difference in the way cookies turn out when you use butter vs. margarine. And never ever use a tub margarine for baking - always choose a good quality stick margarine.

Butter seems to change the consistancy and appearance and the way cookies expand or raise - don't know why. I've used both, and have to say that in this particular recipe, margarine works better. But try it both ways - you can always eat the "failures"~~ I always give DH the "slightly" burned ones, so he's my quality-control checker!! He likes that job -

I also know there is a big difference in regular old chocolate chip cookies when using butter vs. margarine. Perhaps some of the more knowledgeable bakers out there will have a reason - probably due to the lower melting point of butter, or something like that.