Here are some ingredient substitutions for those times that you just do not have what you need.
Baking powder: for each teaspoon of baking powder use; 1/4 teaspoon baking soda plus 1/2 teaspoon cream of tartar, or 1/4 teaspoon baking soda plus 1/2 cup buttermilk (decrease liquid in recipe by 1/2 cup)
Brown sugar: for 1 cup packed brown sugar use; 1 cup White sugar, or 1 1/4 cups confectioners' sugar or 1 cup whit sugar plus 1/4 cup molasses decrease the liquid in recipe by 1/4 cup. If you need to use white sugar it will taste almost the same; white sugar has no real flavor, just sweetness; but it will have a slightly different texture. Brown sugar is now days a blend of white sugar and molasses. It at one time was an unrefined form of whit sugar. Just make sure it gets dissolved and mixed in well to not taste gritty.
Butter (salted): need one cup. use 1 cup margarine or 1 cup shortening plus 1/2 teaspoon salt or 7/8 cup vegetable oil plus 1/2 teaspoon salt or 7/8 cup lard plus 1/2 teaspoon salt
Butter (unsalted): need 1 cup, use 1 cup shortening or 7/8 cup vegetable oil or 7/8 cups lard
Buttermilk: need 1 cup, use 1 cup yogurt (unflavored) or 1 tablespoon lemon juice or vinegar plus enough milk to make 1 cup.
Fats for baking: need 1 cup, use 1 cup applesauce or 1 cup fruit puree
Cream (heavy): need 1 cup, use 1 cup evaporated milk or 3/4 cup milk plus 1/3 cup butter.
Cream of tartar: need 1 teaspoon use 2 teaspoons lemon juice or vinegar.
I hope some of these help some one out. If you need to find another sub. ask and I will see if I have it on my list.