Hi Jared, for our Classic Birthday Cake recipe, we recommend baking in a 9" X 13" cake in a preheated 325°F oven for about 45 to 48 minutes. If you doubled the recipe for your 12" X 18" pan, the batter would actually be a bit thicker, so baking time will likely be longer, but I would check at the time recommended and reset the timer from there. Don't fill the pan more than 2/3rds full or the batter could overflow; you might want to make a few cupcakes with the remaining batter. Alternatively, if you have a scale, you could multiply the weight of each ingredient in the original recipe by 1.85, which would give you a recipe that fits perfectly in your larger pan. In this case, baking time will likely still be a bit longer, but I would still check at the time recommended above and reset from there. If you're wondering how to multiply the 4 eggs by 1.85, we consider one large egg to weigh 50g, so 200g X 1.85 = 370g. Blend 8 eggs together and remove what you need for the recipe. The rest can be saved for scrambled eggs. 

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