Welcome to the Cake Decorators Q&A

Hi team Can you tell me if I need to bake a l2″ square cake longer than a round one of the same size. As I need to use double the mixture to fill the baking tin I wasn’t sure if the timing evened itself out or not

Hello elizabeth_sneddon

Yes the cake will take longer to bake. If you have the timing for a 12″ round, bake to that and then keep an eye on the square cake past that time. Depending on the recipe, density, ingredients etc. you may be looking at an additional 40 -45 minutes or longer.
However that will be for you to keep an eye and nose on.
You can compare how much more batter there is in a square pan to a round pan here:
http://www.cakeflix.com/blog/how-to-work-out-what-size-cake-tin-to-use
Follow 12″ round in the table, run your finger along until you find 12″ square.
Hope this helps.

EDIT

I think possibly 40 -45 minutes will be too much, try 30 mins but do keep an eye on your cake. Use a wooden skewer rather than a metal one to check for doneness.