Some cookie recipes say to chill the dough before baking, and if you think this step isn’t essential, think again.  Chilling the dough helps prevent excessive spreading in the oven. If you’ve ever cut or piped cookies, then admired the perfectly sharp edges only to watch them bake into formlessness, then chilling is the key. It also enhances flavour development, giving you a richer, more complex taste to the finished thing.

Chilling the dough improves texture as well. For example, chocolate chip cookies will bake up crisp on the outside while staying tender or chewy in the middle when they’ve been chilled. The chilling process aids in hydration, where the flour absorbs moisture from the wet ingredients (butter, eggs, etc.), resulting in a more consistent dough and cookies with a tender crumb that won’t turn out dry.

The chemistry behind baking cookies

Here's what happens, especially with high-fat cookies like shortbread:

1. The butter in cookie dough adds richness and flavour. When the cookies bake, the heating butter creates steam, which makes some types of cookies tender. In high-fat cookies like shortbread, the butter helps create that rich, crumbly texture that’s referred to as ’shortness’ in baking terminology. Hence the name. 

2. Sugars: When sugar heats in the oven, it caramelises and gives cookies their sweet, toasty flavour. Sugar also attracts moisture, keeping cookies soft in the middle.

3. Flour: Flour contains proteins that form gluten when mixed with wet ingredients. But in high-fat cookies like shortbread, the butter coats the flour, stopping too much gluten from forming. Less gluten means tender, crumbly cookies, which is exactly what you want in shortbread.

4. Oven Heat: When cookies bake, a few key rethinks happen;

• Maillard reaction: This browns the cookies and adds layers of flavour. (Read about the Maillard reaction here)

• Evaporation: Water in the dough evaporates, causing the cookies to firm up.

• Butter melts: As the butter melts, it creates tiny air pockets, giving the cookies a light, tender crumb.