You put neat balls of dough on the tray and they come out as one thin, greasy sheet. Or the opposite: they sit there as little domes and never spread at all. Both come down to the same thing, which is how long the dough stays soft in the oven before it sets.
My oatmeal cookies are built to stay thick and chewy, so every step in that recipe is about controlling spread. Here is what is going on.
The short answer
A cookie spreads while its butter is melting and its sugar is turning to syrup, and it stops when the egg and flour set. Anything that makes the dough soften sooner or set later makes it spread more: melted or very soft butter, more sugar, or too little flour.
What is going on in the oven
When the dough heats up, two things happen that make it run. The butter melts and coats the flour and sugar, so everything slides. And the sugar dissolves into the water from the egg and the butter, turning into a syrup that makes the dough softer still. Butter melts at a lower temperature than margarine or shortening, so a butter cookie has more time to spread before the egg and flour firm up and hold it in place.
Brown sugar goes further than white. It holds on to water, so it makes more syrup in the oven, which helps the cookie spread and then sets moist and chewy as it cools. White sugar crystallises again as the cookie cools, which is where a crisp snap comes from.
Why cookies spread too much
- The butter was melted or too soft. Melted butter is already liquid when it goes in, so there is nothing to slow it down. My recipe uses butter that is soft enough to beat, never melted.
- Not enough flour. Flour soaks up the liquid and holds the shape, so a dough with a higher share of flour spreads less.
- Bicarbonate of soda. A lot of cookie recipes use it, and it is what gives the wide, thin spread. I use baking powder in my oatmeal cookies for a gentle lift instead, which is why they stay thick.
Why cookies do not spread at all
Everything above, the other way round. Too much flour makes a stiff dough that holds its ball shape. So does dough that went into the oven very cold, because the butter takes longer to melt. If your cookies come out as domes, press the balls down a little before they go in.
Should you chill the dough?
Chilling firms the butter, so the dough takes longer to soften in the oven and spreads less. It is useful when a dough is very soft. My oatmeal cookies do not need it, because the soft butter and the baking powder already keep them thick, so I bake them straight away.
If they have already spread
They still taste good. While they are still hot and soft on the tray, you can push the edges back in with a spatula or a round cutter to tidy the shape. Thin cookies also make a very good crumb for a cheesecake base.
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