One of the most common questions I get has to do with cake recipes or variations on cake recipes.  Some examples of the questions I receive are:

- What is the correct ratio of baking powder to baking soda for a cake?
- How much baking soda should I use per cup of buttermilk or sour cream?
- Should I use baking powder or baking soda?

One of these I have answered already.  All other things being equal, the amount of baking soda to use per cup of buttermilk is about ½ teaspoon.   All of these questions, however, are related.

For a simple cake recipe, you want to use about 1 to 1 ¼ teaspoons of baking powder for each cup of flour.  That is the starting point.  Now, suppose you use buttermilk in place of milk in the recipe.  If the recipe calls for ½ cup of milk, which you replace with ½ cup of buttermilk, then you need ¼ teaspoon of baking soda to react with the acidity of the buttermilk.

If you just add the baking soda to the recipe, then your cake may have too much leavening, which may make it rise too much, split, or even fall.

That means you need to reduce the baking powder by the equivalent amount of leavening.  In Baking Soda vs Baking Powder, I  noted that the formula for equivalence is 1 teaspoon of baking powder can be replaced with ¼ teaspoon of baking soda and ½ teaspoon of cream of tartar, with ¼ teaspoon of cornstarch.  In this case, the cream of tartar is used to provide the acidity to react with the baking soda.  The cornstarch is added simply to make up the same volume and has no real role in the mixture.

So, back to the cake recipe, if you replace ½ cup of milk with the same volume of buttermilk, you need to replace 1 teaspoon of the baking powder with ¼ teaspoon of baking soda.  If the recipe only uses 1 teaspoon of baking powder, you will replace it all and the recipe only needs baking soda.  If it uses more, then you have to use both baking soda and baking powder.

Now, suppose you are making a chocolate cake that uses cocoa powder.   If you are using cocoa powder that is not "dutched", it is acidic, so you need to replace more of the baking powder with baking soda.  If you don't, the cake may turn out a light reddish brown color.  Cocoa powder that is dutched has an alkaline ingredient added to neutralize the acidity and darken the color, so the amounts of baking soda and baking powder do not need to be adjusted to compensate.

If, however, you were making a ginger cake, then you would need to replace baking powder with baking soda, since the molasses used in ginger cakes is acidic.  The exact amount you would need to replace would depend on whether you are using fancy or cooking molasses in the recipe, as they vary in their acidity.

Now, just to make things more complicated, eggs are a natural leavener.  If they make up the largest proportion of the liquid in a recipe, then it is entirely possible that you don't need to use baking soda or baking powder unless, of course, you are adding an acidic ingredient that needs to be counteracted.

All of this changes again if you live someplace where altitude can affect your result.

In short, creating a recipe from scratch can be a difficult problem involving a detailed understanding of the chemistry of food and a lot of trial and error.  The easiest approach is to look in cookbooks for recipes that come close to what you are attempting, and then adapt them by making small changes.  If you want to make a coffee cake, look for similar coffee cake recipes as a starter.  If you are planning something with the consistency of a pound cake, start from there.  In any case, plan to try a few times before you get the result you are looking for.