From Linda Larsen,
Your Guide to Busy Cooks.
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Bar cookies are similar to individually baked cookies, with some changes. Bar cookies can be cake-like, with a bottom crust and a filling, two-crust with a filling in between, no-bake, or pastry-like. Here's what the ingredients do in a bar cookie.

Flour

Flour provides the structure for the product. The gluten, or protein, in flour, combines to form a web that traps air bubbles and sets. Starch in flour sets as it heats, to add to and support the structure. In bar cookies, we want little gluten formation, which makes products tough. Fats and sugars help prevent gluten formation. In most bar cookies, all-purpose flour is a good choice; it has less gluten than bread flour. For many bar cookie recipes, I like to reduce the amount of flour by 1-2 Tbsp. per cup used for a more tender product. If you use too much flour, the bar cookie will be tough and dry.

Fat

Fat coats gluten molecules so they can't combine as easily, contributing to the finished product's tenderness. When sugar is creamed with fat, small pockets of air form from the sharp edges of the crystals interacting with the fat. These pockets fill with carbon dioxide from the leavener, if any, and form a finer grain in the finished product. Fats also carry flavors and add to a tender mouth-feel.

Sugar

Sugar adds sweetness, as well as contributing to the product's browning. Sugar tenderizes a bar cookie by preventing the gluten from forming. Sugar also holds moisture in the finished product. Brown and dark brown sugars also add a richer flavor. Honey, corn syrup, and molasses add moisture and additional flavor.

Eggs

Eggs are a leavening agent and the yolks add fat for a tender and light texture. The yolks also act as an emulsifier for a smooth and even texture in the finished product. Follow directions carefully for using eggs in bar cookies. If the recipe calls for separating the eggs and beating the yolks and whites separately, separate the eggs while cold, and let the whites warm for about 1/2 hour before beating them. This will result in higher volume, since the egg white proteins are more relaxed.

Liquid

Liquid helps carry flavorings throughout the product, forms gluten bonds, and reacts with the starch in the protein for a strong but light structure. Liquids also act as steam during baking, contributing to the tenderness of the product.

Leavening Agents

Baking soda and baking powder form CO2, that is held by fat pockets, gluten and starch, which makes the baked product rise. Baking soda and powder are not interchangeable; be sure that you have the product the recipe calls for. When a batter or dough is acidic (from brown sugar, sour cream, buttermilk), most recipes call for using baking powder and baking soda. The soda neutralizes the acidity so the product tastes balanced.

Now that you know how to bake the best bar cookies, here are lots of recipes: Bar Cookie Recipes