It's difficult to bake cookies that are consistent from batch to batch. Cookies are basically flour, butter or shortening, sugar and flavorings. Altering these ingredients or the baking temperature changes a cookie's look and texture.
Here's what I do to help guarantee more consistency:
1. Make sure your cookie dough is cold before placing it in the oven. I like to use all-butter dough in my baking -- I like the taste and I like to bake with all-natural ingredients. However, the use of butter in cookie baking will make for a less consistent cookie. If the batter is very soft, so is the butter in it -- the cookie will spread immediately when placed in a hot oven and will come out flat.
To overcome this, I ALWAYS place the cookie dough on a prepared cookie sheet and freeze both for 5 to 10 minutes (play around with the times) before baking. When you place the frozen cookie sheet and dough in the oven immediately, the fat will stay cold longer and the cookie will retain its shape better. Have you ever noticed that icebox-type cookies are very consistent?
Some bakers like to use a combination of 50 percent butter and 50 percent shortening. Shortening is not as sensitive to shifts in temperature and the butter will give the cookie flavor. Cookies made with shortening will not spread as much and are more consistent.
2. Use the same type and brand of flour: Be consistent with all your ingredients, but especially flour. Flour, depending on the type, when and where it was milled and the amount of humidity in the air, will react differently in each baking batch, even with the same recipe. I bake all of my cookies with the one type called for. I do not even switch brands.
3. Measure the wet ingredients very carefully: Fluctuations will cause a cookie to spread or puff. A puff is caused by too little liquid in the cookie recipe. A little too much liquid and the cookie will spread. Even the addition of a jumbo egg versus a large egg will change the whole look and texture of the cookie.
Adapted from The Healthy Oven Baking Book, by Sarah Phillips (Doubleday, 1999).