I discovered a couple recipes on pinterest recently. I made these http://fakeitfrugal.blogspot.it/2012/04/fake-betty-crocker-brownie-mix>brownieslast night and they were great. You could make up several bags of the dry ingredients since you have them out and add the wet ingredients when you want to bake them. The blog post states that the cost per bag is $0.30 compared to over $2.00 for the mixes you buy in the store.
Here is a Peanut Butter cookie recipe with 4 ingredients. I haven’t tried them, but the Pinterest comments said they are supposed to be the best.