- They spread. As the dough warms, the butter begins to melt, so the dough slackens and begins to spread outwards.
- The edges set. As the dough spreads, the edges become thinner, exposing them to the full heat of the oven while they encounter the hotter areas of the pan as they spread outwards. Thus, they set long before the centre does.
- They rise. As the butter melts, the cookie’s structure loosens, so that the water in the dough is able to combine with the baking soda, dissolving it. The baking soda then reacts to the acidic components present in brown sugar, creating gases that cause the cookie to rise.
- Egg proteins and starches set. When the cookies get hot enough, the egg proteins and starches begin to set, finalizing the shape of the cookies.
- Sugar caramelizes. The edges and bottom of the cookie, the hottest areas of the cookie, begin to brown as sugar granules melt together and caramelize.
- The Maillard reactions takes place. The Maillard reaction is a chemical reaction between amino acids and reducing sugars, so along with the caramelizing sugar, proteins in the cookie begin to brown, producing a rich, nutty, toasted flavour. This is the same reaction that occurs in bread and seared steak.
- They cool. The action doesn’t stop when your cookies come out of the oven. As they cool, the liquified sugars cool and harden, producing crisp bottoms and edges, and the air inside cools, causing the cookie to deflate slightly.