Homemade Baking Powder
(Single Acting)
2 Measure cream of tarter
1 Measure baking soda

Mix together and use immediately. If you want to mix ahead of
time and store no longer than one month, add 1 measure cornstarch

Note: I use teaspoons for measure amount, but it really depends
on how much you need. The cornstarch will help keep the other
ingredients from absorbing moisture (this is what is used in
manufactured baking powder). If you have a recipe that calls
for double acting baking powder, just use twice the amount.

Oh, one more thing, to check and see if the baking powder in
your pantry is still active, stir 1 teaspoon into one-third cup
hot water. There should be immediate vigorous bubbling. If no
bubbling or bubbling is sporadic, the baking powder is past its
prime.

Homemade baking powder starts to fizz and release carbon dioxide
the minute it is added to liquid, it is very important that the
oven be preheated and baking pans be ready before mixing batter.
Mix quickly and put in oven right away.