Baking powder . . .

I do not bake often enough to buy baking powder premixed. Years ago I found a recipe in Mary Webber's Frugal Family's Kitchen Book. 

Mary's substitution recipe is:
in place of 1 teaspoon of baking powder use  1/3 teaspoon of baking soda and 1/3 teaspoon cream of tarter. (I do not add cornstarch).

If you do use baking powder perhaps this article would interest you!

Why You Should Use Aluminum-Free Baking Powder