Corn-Free Baking Powder from The Complete Food Allergy Cookbook by Marilyn Gioannini
"It is easy and economical to make your own baking powder. The basic ingredients are cream of tartar and baking soda. Cream of tartar is an acid, and baking soda is a base, and when they are mixed with liquid, bubbles form. Arrowroot powder is added to help keep it free-flowing. If the mixture cakes, mash it with your finger in the measuring spoon.
"To make your own corn=free baking powder, mix together 2 parts cream of tartar, 1 part baking soda, and 2 parts arrowroot powder. Store in an airtight container, and substitute in any recipe calling for baking powder. It is more economical to buy cream of tartar and arrowroot powder at a natural foods store, especially if they are sold in bulk.
"This is a single-acting baking powder. That is, all of the rising occurs as soon as the liquid is added to the dry ingredients. For best results, mix all of the dry ingredients well, mix the wet ingredients separately, and have the pan and oven ready to go before mixing them together. Keep mixing to a minimum."
I believe you can also use tapioca flour, potato flour, or another kind of starch instead of the arrowroot.
Or you can just leave out the starch altogether and substitute 1/2 tsp. cream of tartar and 1/4 tsp. baking soda for each teaspoon of baking power called for in the recipe.
If you are allergic to grapes or glutamates, you should be aware that cream of tartar (a.k.a tartaric acid) is a grape-derived acid salt that is a byproduct of the fermentation in wine-making.
You can often substitute vinegar (but not white vinegar if you're allergic to corn) , lemon juice or any other edible acid for cream of tartar in recipes. You'd use 3 parts vinegar or fresh lemon juice to substitute for 1 part cream of tartar, and would probably need to lessen the amount of other liquids in the recipe accordingly.