If you've ever had the one batch of cookies emerge from the oven perfectly, only to have the second batch turn out flat and disappointing, don't blame the recipe — it might actually be your baking sheet.
Specifically, the problem might be your hot baking sheet. Dropping cookies onto a hot baking sheet — one that just came out of the oven and hasn't had a chance to cool down — means that the butter begins melting even before the cookies go into the oven. These cookies ultimately spread out too much during baking. At best, you get some flat and crispy edges, but at worst, your perfect sugar cookie hearts will melt into amorphous blobs and your chocolate chunk cookies will turn into wafers.
The solution is easy: Just let your cookie sheets cool for a few minutes before starting another round. These thin metal sheets cool very quickly on top of a cooling rack or even laid over the top of your sink — wherever you can get a little airflow underneath the baking sheet to help cool it down. Once it is room temperature again, the baking sheet is ready to use.
It's helpful to have a second baking sheet in the rotation so you can bake with one while the other is cooling. If you don't, you can scoop your cookies out onto a piece of parchment while the baking sheet is cooling, then lift the cookies onto the baking sheet using the parchment. If you need to work even more quickly, run your baking sheets under cool water and then wipe them dry before using.
Just the extra few minutes you take to cool down your baking sheets can make a real difference when it comes to making your last batch of cookies turn out just as beautifully as your first.