Warm dough and soft butter are the biggest reasons cookies spread during baking, according to baking guidance published by Smitten Kitchen. The site's author, responding to a frequent question in recipe comments, writes that dough which is too warm or soft will spread more than cooler dough, and recommends refrigerating dough for 15 minutes or longer before baking in a warm kitchen.
Butter temperature is a major culprit. When a recipe calls for "softened" or "at room temperature" butter, the target is butter that takes a fingerprint when poked but does not hold the impression. A baking sheet still warm from a previous batch will also encourage cookies to spread before they begin to bake.
Temperature is not the only factor. A greased cookie sheet promotes spreading; the source suggests flouring it after greasing, or using silicone paper or a Silpat mat instead. Because sugar liquefies as it heats, a cookie with more sugar and less flour or fat is more likely to spread than one with a lower sugar proportion.
How mixing and ingredients affect spread
When creaming butter and sugar, the source advises beating just long enough to combine the ingredients — about 30 seconds on an electric or stand mixer, citing baker David Lebovitz — to avoid whipping in too much air, which expands as bubbles steam in the oven. Cakes, by contrast, have no such limit on airiness.
At higher altitudes, cookies made with baking soda tend to spread more. Fat type also matters: butter, which melts at body temperature and is nearly one-fifth water, spreads more than margarine, and both spread more than shortening. All Smitten Kitchen cookie recipes use butter, the source notes, making temperature control of the butter, dough and baking sheets especially important.
Editorial Notes