From the baking faq thread,
How do I convert cupcakes into cakes?
One batch of cupcake batter makes 1 round cake (either 8" or 9"). If you want a thick 9" cake, then use 1.5 batches of cupcake batter. Bake for 10 - 15 minutes longer than you would bake the cupcakes for, and test for doneness - you want to make sure they're cooked all the way through.