A common mistake that I still seem to do, is to add too much flour by simply over-filling your cup measure. Be sure when using a cup measure, dip and level off with the edge of a butter knife; and do not tap the cup because that results in the flour settling, But what about the sugar cookie makers? When rolling out your dough use powdered sugar instead of flour to avoid sticking. Flour will only contribute to crumbly,dry hard cookies.
Another problem is cookies that bake unevenly. Be sure that you always preheat your oven before you place you cookies in, centering them in the middle of the oven. Avoid baking your cookies for longer than the recommended time, and do not allow them to cool for too long on the cookie sheet, because the cookie sheet is still very hot and can continue to cook the cookie after removing from the oven. So err on the minimal cooking time stated.
Still further often times it is the ingredients that you use and how you mix them together. Fats are very important in the production of cookies. What this means is more fat(butter,shortening,etc) equals a flatter crisper cookie. Less fat results in puffier more cake-like textured cookie. The leaveners(baking soda and baking powder) can affect the final product. Baking soda neutralizes the acid in the dough causing the cookie to brown , while baking powder, that already contains an acid results in cookies being puffier and lighter in color. Sugar like fat melts in the oven and can affect your cookie. White sugar in cookies tends to be a little more crispier that it's cousin brown sugar,brown sugar absorbs moisture, making the cookie chewier. Eggs, these aid in the rising of the cookie and produce that cake-like texture, the yolk can act as a binder making the dough richer, the egg white will make the cookie dry, to counterbalance that in the case of macaroons extra sugar is added.
When mixing the dough remember to not over mix the dough, that will result in a flat lifeless cookie. When making drop cookies like chocolate chip be sure during the process of mixing that you scrape the bottom real well, because some mixers can not pull from the bottom and sides of the bowl leaving a portion of the dough with unincorporated fat. This causes some of your cookies to spread right out and be crisp.
A few tips for the cookie maker: Darker the pan the faster it will heat , dark absorbs heat so the bottom of the cookie will cook before the top. Don't crowd your pan, and don't keep opening the oven to peek, this lowers the temperature, and then the oven fights to maintain temperature. Chill your dough especially for sugar cookies. The fat does not melt as fast while you are in the rolling and cutting phase. And last but not least, don't eat all the cookie dough, save some for cookies.
Strength is the capacity to break a chocolate bar into four pieces with your bare hands- and then eat just one of the pieces.~Judith Viorst