Can anyone help me? I am doing an engagement cake for my sister in about 3 weeks and she wants a two tier sponge cake, 12" and 10". my problem is I like the 'one bowl sponge' recipe in the ISS book but it only goes up to 9" round. Does anyone know how to change the ingredients to fit my tins? thanks and hope someone can help. 
Here's a link to the Wilton site that will tell you how many cups of batter you need for each size.
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
thanks to everyone for all your advice, my sisters cake turned out really well I think (for a first time, stacked cake anyway) 
i've already put a photo on the web of it (its the purple and white cake ) , i couldn't wait!! hopefully it looks ok. please feel free to let me know if you think there is anyway i could have improved it, any tips or comments are greatly welcomed. 
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