If you don't have an 8 x 8 inch (20 x 20 cm) square baking pan, you can use a 9 inch (23cm) round tin instead. If you have a 9 x 9 inch (23 x 23cm) square baking pan, please note that this will be too big and your cake will be quite thin. It will still work, but you'll have to start checking for doneness earlier.