Do you feel frustrated when your home-baked cookies/biscuits don’t turn out as you expect? Why are Cookies (called biscuits in Australia) sometimes too hard, too soft, way-too-spread-out, or hard enough to use as a cricket bat?

My investigations into this blight on the Home Baker led me to conclude that baking is a science, and pastry cooks and chefs who are required to replicate the exact same foods with the exact same textures and tastes every single time, have my endless admiration. For the path to creating the perfect biscuit is laden with pitfalls, and endless variables that are bound to confuse, frustrate and annoy the most patient and placid of us.

Not only do you have to achieve consistency at technique, control the uncontrollable variations in oven temperature and heat distribution, you also have to conquer such variables as appropriate shelf height and heat setting in multi-functional ovens, incorrect weighing/measuring of ingredients, the endless debate on whether to fold or beat, cover or uncover the cooked item, and the list goes on.

Something as simple as using low-fat butter or milk can drastically alter results. Nevertheless, it is useful to consider why things may have gone wrong. http://www.sunset.com had some answers for me:

What makes cookies soft and chewy?

High moisture content does; so the recipe, baking time, and temperature must be adjusted to retain moisture. Binding the water in butter, eggs, and brown sugar (it contains molasses, which is 10 % water) with flour slows its evaporation. The dough needs a little extra flour, which makes it stiffer. The stiff dough spreads less, less liquid evaporates, and the cookies are thicker.

Mass also helps cookies stay moist–big dollops of dough make softer and chewier cookies than tiny spoonfuls of dough. Bake these thick cookies for a shorter time at a high temperature to firm them quickly and minimise spreading. Most important, don’t bake them too long–remove them from the oven when the cookie rim is brown and at least 1/3 of the centre top remains pale. The cooked centres will be soft.

Why are some cookies cake-like instead of chewy?

A little extra liquid in the cookie dough from water, egg, or milk makes the dough more elastic and adds steam as the cookies bake, making them puff more.

What makes a cookie crisp or crunchy?

Reducing the amount of ingredients that hold moisture–flour, egg, and brown sugar–makes it easy for liquid to evaporate, producing crisp cookies. The fat, which goes up proportionately when other ingredients are cut back, gets hotter than the water in the dough and drives out the moisture. Fat also makes the dough softer and melts when hot, making the cookies spread. For crispness, bake cookies longer at a lower temperature to give them more time to spread before they firm. Then bake long enough to dry and brown them evenly to develop the maximum toasty flavour and crisp texture throughout.

What else makes cookies spread as they bake?

Having trouble with a favourite recipe? Cookies are suddenly spreading excessively?

• Low-fat butter or margarine spread, which has about 20% more water, used in place of regular butter or margarine is often the culprit. Low-fat products can’t be used interchangeably with regular fats for baking without recipe adjustments.
• Cookies also spread when you drop high-fat dough onto a hot baking sheet; the heat melts the dough, and cookies spread before they’re baked enough to hold their shape.

Some chocolate chip cookie recipes turn out crunchy. Others are chewy. Why?

The way they measure ingredients and the real temperature of their ovens are the usual reasons cooks get different results from the same recipe.

Flour should be stirred to loosen and fluff it, then spooned gently into a dry-measure cup (the kind you fill to the rim), and the top scraped level. If you tap the cup or scoop flour from the bag, the flour gets packed down, and you can easily add 2 to 4 extra tablespoons flour per cup.
You can scoop up white sugar; it doesn’t pack. But you should firmly pack brown sugar into a dry-measure cup and scrape the top level.

Dry ingredients should not be measured in heaped-up cups or spoons; scrape dry ingredients level with the surface of the measuring tool.

Measure liquid ingredients with liquid-measuring (usually glass or plastic) cups.

Sunset.com

It might be prudent to double-check your oven is heating correctly with a thermostat. Adjust if your oven is overly hot or cooler than it should be.

Controlling Spread in Cookies with Baking Soda: Cookies spread across a cookie sheet when they have too little structure and cannot hold their shape. Whether this is desirable or not depends on what kind of cookie you wish to bake. There are many ways to increase cookie spread: One way is to add a small amount of baking soda, as little as .25 to .5 ounces (5 to 15 grams) for 10 pounds (4.5 kilograms) of cookie dough. This increases the pH of the dough, weakening gluten, and also weakening egg protein structure. With less structure, cookies spread more and have a coarser, more porous crumb. Since moisture evaporates from a porous crumb more easily, baking soda often provides for a crisper crumb, as well. Measure baking soda carefully. Baking soda increases browning significantly, and if used at too high a level, it leaves a distance salty-chemical off flavour. When working at high altitudes, omit baking soda from the cookie dough. The lower air pressure at high altitudes already encourages spread.

How to Ensure Baking Success in Using Ingredients

• Check the expiry date on egg carton and other ingredients too.
• Eggs should be at room temperature. The emulsion can be ruined if eggs or other liquids are too cold or too hot when they are added.
• Measuring Flour: Too much flour can make some cookies rock-hard. When in doubt, err on the side of less flour. Use a scale if the recipe offers a weight equivalent. Spoon the flour into your measuring cup and sweep a spatula across the top to level it off. Don’t use the measuring cup as a scoop, or it’ll pack the flour, and you’ll end up with more flour in the cup than intended.
• Nuts:  Smell and taste nuts before using. Oils in nuts can turn rancid quickly. Store any leftover nuts in the freezer for longest shelf life. 
• Butter:  Make sure your butter is at room temperature, otherwise it won’t cream properly with the sugar. The terms “room temperature,” “softened” and “soft” mean different things. The temperature of the butter can make a difference in the recipe. Most cookie dough recipes depend on the emulsion that occurs when you cream butter and sugar together. This emulsion will not happen if the butter is too hot or too cold.
• Room Temperature Butter: It should be pliable enough that your finger can leave a mark in it, without being soft and greasy. Set the butter out at least one (1) hour in advance.
• Softened Butter: Will feel a little warmer to the touch, and it will be much easier to leave a deep indentation, but it should still be firm enough to pick up without falling apart.
• Soft Butter: Will be too soft to pick up.
• Microwave Butter: Do not try to microwave your butter as it will just end up too soft. If you don’t have an hour’s lead time, increase the surface area by cutting the butter into small pieces or shredding it on the large holes of a grater. It will then come up to temperature in approximately 10 minutes.
• Unsalted Butter: Unsalted butter is generally recommended because some salted butters have more sodium than others.  Do not use low fat butter/margarine. Low fat margarine has 20 % more water.
• Salt:  Use the full amount of salt called for in a recipe, especially is using unsalted butter. If you use salted butter, only use 1/2 the amount called for in the recipe. Don’t skip the salt, as salt brings out flavours and balances the sweetness in a recipe.
• Sugar: The type of sugar used in your cookies can promote spread in baked cookies. To understand this, you need to know that sugar is a tenderiser which interferes with the formation structure. Sugars with a finger granulation promote more spread, (probably because they dissolve sooner, and only dissolved sugars will tenderise). Powdered sugar (confectioner’s sugar), when it contains cornstarch, prevents spread in cookies despite it finer grind.

Source: whatscookingamerica.net/Cookie/CookieTips.htm