Yes, you can double the mix in a 9" X 13" baking pan, although the batter will be thicker in the larger pan, so baking time will be a bit longer than the times listed, and you will also need to adjust the baking temperature depending on the type of pan (glass, or light or dark metal) you're using. I would recommend checking at the time recommended on the package, and then resetting the timer from there. Baking times can vary considerably, depending on the pan. Use these guidelines for best results when baking a single mix: In darker pans, including our corrugated King Arthur Flour USA pans, bake for 30 to 35 minutes. In lighter-colored pans, bake for 35 to 40 minutes. For glass pans, decrease the oven temperature to 325°F, and bake for 40 to 45 minutes.When done, the brownies' internal temperature should register 214°F to 218°F on an instant-read thermometer. A toothpick inserted in the center may come out with a few moist crumbs clinging to it; it doesn't have to be absolutely clean.

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