THE PROBLEM: It always used to happen that my biscuits and cookies would have over-baked bottoms. Using parchment paper helps a little (and helps a lot with cleanup) by insulating a little from the hot baking sheet. The bottom-line is that all ovens heat from below. So even with my oven’s convection fan, the part of the oven below the baking sheet is always hotter than the top. Previous, the only tool in my tool chest to prevent it was to lower the overall oven temperature. But the recipe calls for a specific temperature for a reason, and lowering the oven temperature will almost always have unwanted consequences. For example, “oven spring” usually calls for higher temperatures to cause rapid rising of the leavening agent (yeast or baking soda/powder) before the flour sets. A lower temperature will result in denser biscuits and cookies.
THE SOLUTION: Half-way through baking, put a large Pyrex casserole dish filled with 1/2″ to 1″ of hot tap water on the shelf below whatever your baking. (See photo below). This technique allows time for the “oven spring” to occur, but then prevents the metal sheet pan from overheating and burning the bottoms. The exact timing will depend upon the characteristics of your individual oven, but I have found that half-way is the general rule for my oven.
It’s definitely a balancing act; too soon and the bottoms wont brown, and too late and the bottoms will overcook. If your oven requires you to include from the beginning, be sure to include the water during the entire preheating cycle. The idea is not to lower the overall oven temperature, but rather to even out the temperature in the top and bottom of your oven.