Don't panic - just substitute. (Well, you can still panic if it will make you feel better, but it's really not necessary). Instead, just use 1/4 tsp. soda plus 1/2 tsp cream of tartar, or 1/4 teaspoon soda plus 1/2 cup sour milk or buttermilk.
And speaking of which, what do you do if you're out of buttermilk?
Here are some of the substitutions I've found most helpful.
- Instead of one cup of buttermilk, add 1 tablespoon vinegar to slightly less than one cup of milk.
- Instead of 1 cup of cake flour, try 7/8 cup all-purpose flour plus 2 Tbsp. cornstarch
- Instead of 1 tsp. dried leaf herbs, try 1/4 tsp. powdered herbs
- Instead of 1 cup of honey, use 1/ 1/4 cups sugar and 1/4 cup of liquid
- Instead of 1 Tbsp. cornstarch, use 2 Tbsp. flour or 1 1/3 Tbsp. quick-cooking tapioca.
What substitutions have you used in a pinch?