Dread Lörd Kaolian wrote:
Recopies that combine baking soda and baking powder are usually made by people that don't exactly know what one or the other actually does.
My understanding is that sometimes you need a higher ratio of soda to acid in order to get the correct rise effect. I also vaguely recall reading something about differences between rising versus baking and the different effects of each during those stages. Mostly bread related though, not cookies. I think nearly every recipe I've seen for some kind of bread asks for both soda and powder, so I doubt they all just got it wrong.