Hi Joan, if your recipe includes weight measurements then I would multiply each ingredient weight by 1.44 to get the larger recipe, suitable for your 8" X 11.5" pan. Unfortunately, this won't work quite as well when you're measuring by cups. If you are working with cup measurements, I would suggest multiplying the recipe by 1.5, and then fill the larger pan just to the level you normally fill the 8" X 8" pan. You'll likely have a little extra batter, but this will make the math much easier! If you decide to use all the batter in the larger pan (which may work fine), baking time may be a bit longer, but check at the usual time recommended, and then reset the timer from there. 

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