Why do cookies sometimes come out flat instead of thick and chewy?

Cookies often come out flat because the fat in the dough melts too quickly, causing the cookie to spread before the structure has a chance to set in the oven.

Here are the most common reasons this happens:

• The butter was too soft or melted: If you use butter that is completely melted or overly softened, it doesn't hold air bubbles when creamed with sugar. Start with cool, room-temperature butter that still holds its shape when pressed.
• Too much sugar or fat: Sugar liquefies as it heats. If your recipe has a high ratio of sugar to flour, the cookies will spread thin before the flour can structure them. Similarly, too much butter creates the same effect.
• The dough wasn't chilled: Chilling the dough for at least 30 minutes before baking solidifies the fat. This ensures that the cookie keeps its shape longer in the oven, allowing the edges to set before the center collapses.
• The baking sheet was too hot: If you put your next batch of cookies onto a baking sheet that is still warm from the previous round, the butter will melt instantly. Let your pans cool down completely between batches.
• Not enough flour: It is easy to accidentally under-measure flour. If the dough is too wet, it won't have enough structure to stand tall. Use a kitchen scale if possible, or use the "spoon and level" method rather than scooping the flour directly with the measuring cup.
• Oven temperature: If your oven is too cool, the fat will melt and spread before the heat is high enough to "set" the cookie. Ensure your oven is fully preheated before putting the tray in.