Uh oh, I thought I had some...

How many times have you started a recipe and THOUGHT you had everything and realized that you were missing one ingredient? Can I leave it out? What can I substitute? If you are baking, no way can you leave it out....baking is a science...your recipe won't work.

So I listed some common things that we bake with that you may need to substitute in a pinch...

Baking Mix (2 cups) - Mix 1 3/4 cups all purpose flour, 2 1/2 tsp baking powder, 3/4 tsp salt and 1/3 cup shortening

Baking Powder (1 tsp)- 1/4 tsp baking soda plus 1/2 tsp cream of tartar OR 1/4 tsp baking soda plus 1/2 cup sour milk, buttermilk, or plain yogurt OR 1/4 tsp baking soda plus 1/2 tsp vinegar or lemon juice used with sweet milk to make 1/2 cup

Buttermilk (1 cup) - Mix 1 tbsp white vinegar or lemon juice with 1 cup milk, let stand 5 minutes OR 1 cup whole milk, 1 3/4 tsp cream of tartar OR 1 cup water, 1/4 cup buttermilk powder OR 3/4 cup of plain yogurt and 1/4 cup of milk OR 1 cup plain yogurt (for baking) OR
1 cup sour cream (for baking)

Cake Flour (1 cup)- 1 cup minus 2 tbsp all-purpose flour, sifted OR 1 cup all-purpose flour sifted 3 times, then measured to make 1 cup OR 7/8 cup all-purpose flour plus 2 tbsp cornstarch

Chocolate (1 square unsweetened) - 3 tbsp unsweetened baking cocoa plus 1 tbsp butter, vegetable shortening or margarine OR 3 tbsp carob powder plus 2 tbsp water

Cocoa (1/4 cup or 4 tbsp) - 1 oz (square) chocolate (decrease fat called for in recipe by 1/2 tbsp)

Cornmeal- self rising (1 cup) - 1 cup plain cornmeal, 1 tsp baking powder, 1/2 tsp salt

Corn Syrup (1 cup light) - Mix 1 cup granulated sugar with 1/4 cup liquid used in recipe OR 1 1/4 cups light brown sugar, 1/3 cup liquid OR 1 cup honey

Corn Syrup (1 cup dark) - 1 cup light corn syrup OR 3/4 cup light corn syrup plus 1/4 cup molasses OR 1 cup maple-flavored syrup

Cream, half and half (1 cup) - 7/8 cup whole milk plus 1/2 tbsp butter OR 3 tbsp oil plus milk to equal 1 cup OR 1 cup evaporated milk OR 3/4 cup whole milk plus 1/4 cup heavy cream OR 2/3 cup skim or low-fat milk plus 1/3 cup heavy cream

Cream, heavy (1 cup, not for whipping) - 3/4 cup sweet milk plus 1/3 cup butter OR 2 cups whipped dessert topping OR 1 cup evaporated milk

Crème fraîche (1 cup) - 1/2 cup sour cream plus 1/2 cup heavy cream

Cream of tartar (½ tsp.) - 1 1/2 tsp lemon juice or vinegar

Dark brown sugar (1 cup) - 1 cup granulated sugar plus 2 tbsp molasses or dark corn syrup.

Egg (1) - Mix 2 tbsp of milk with 1/2 tsp baking powder OR 2 egg yolks OR 1 egg white plus 2 tsp oil OR 2 egg whites OR 1/4 cup cholesterol-free egg substitute OR 2 egg yolks (for puddings or custard) OR 2 egg yolks plus 1 tbsp water (for bars or cookies)

Onion Soup Mix (dry 2 tbsp) - 1 tbsp instant minced onion, 1 tsp dry beef bouillon, 1/4 tsp. sugar

Powdered sugar (1 cup) - 1 cup granulated sugar and 1 tsp of cornstarch finely ground in blender.

Sweetened condensed milk (1 cup) - 1 cup nonfat dry milk, 1/2 cup boiling water, 2/3 cup sugar, 3 tbsp melted butter (process in blender until smooth)

Yogurt, plain - Sour cream OR Buttermilk OR Cottage cheese blended until smooth

Of course, the site I got this from had gadzillion other substitutions, if you are looking for something not on my list, check them out:

http://www.recipetips.com/kitchen-tips/t--200/food-substitutions.asp

Recipe tips has many many more suggestions:
Conversion calculators, How to much to buy, can equivalents, alcohol substitutions, cooking times, etc.