What’s the size as your “normal” square pan? Once we know this number, it will be easy to calculate the amount of batter your need, by using volume calculation.
Or else, just fill your “normal” pan with water and then into the 12×18 pan. Count how many times you do this. That’s exactly the factor of batter you need.
(Say, if you can fill your 12×18 pan by filling your normal pan three times, then you need to mix 3x your usual amount).
For baking, i suggest lowering the temperature of the oven by 10-20 degrees C, and a longer baking time, up to 30-60 minutes longer.
Xuân-Minh, Minh Cakes || http://www.minhcakes.ch/en || http://www.facebook.com/minhcakes.ch
