Engineering the Perfect Cookie

Engineering the Perfect Cookie
Baking cookies can be quite scientific.  When you put raw cookie dough in the oven, you’re really just setting off a series of chemical reactions.  
Here are the key moments of the baking process:

1.The dough spreads: the butter inside the cookie dough begins to melt so that the perfectly formed dough ball you put in the oven begins to flatten out.  The final diameter of the cookie is determined in this process.

2.The rise: At about 212°F, the water in the dough begins to turn into steam. This steam expands the dough and it rises. The baking soda does a similar reaction by creating carbon dioxide.  This makes the cookie flaky and light.

3.Color and flavor:  As sugars in the dough break down, they transform from clear, odorless crystals into a brown, fragrant liquid that’s overflowing with aromas and tastes (such as butter scotch, sweet rum and popcorn). The second yummy process packs the cookie with even richer tastes. The reaction involves not only the sugars in the dough but the proteins from the egg and flour as well. So it churns out toasty, nutty and even savory flavors. The Maillard reaction also helps to darken the cookie’s surface.
More about Maillard reaction:
http://en.wikipedia.org/wiki/Maillard_reaction


You can adjust the ingredients to refine the cookie’s architecture.  
"If you start off with melted butter in the raw dough — instead of cold butter chunks — the dough is immediately wetter and will spread out faster," explains UCLA’s Liz Roth-Johnson. This makes a flatter, wider cookie.  But using melted butter will also make the cookie chewier: as the water in the butter converts into gas, it creates air pockets — the melted butter creates smaller holes.   

Source and further reading:
http://www.npr.org/blogs/thesalt/2013/12/03/248347009/cookie-baking-chemistry-how-to-engineer-your-perfect-sweet-treat?ft=1&f=1007