How long should I bake a 13" (330 mm) sponge cake?
Which gas mark should I use?
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A 13" cake is large enough to be a major problem -- the center just won't set correctly before the edges overcook.
You generally want to cook large cakes at a lower temperature than you'd cook a smaller cake at; they won't rise as much, but as they're typically used for stacking, a denser cake may be preferred.
There are also two gadgets that you may wish to consider when you're dealing with cakes over 10 inches:
I can't help with the gas mark, as I don't know the British system.
The recipe should specify the temperature, as it can vary depending on the ingredients and the desired texture. If I had to guess, I'd use a temperature of 180°C (350°F), or gas mark 4. However, some recipes call for 3 or 5. (Growing up in the US, I had never heard of "gas mark" until just now! Our ovens are usually marked in degrees Fahrenheit.)
As for how long, this tends to be an easy one for most cakes: once it starts to look done (it's starting to turn evenly brown on the outside) stick a toothpick in it. If the toothpick comes out clean and dry (perhaps a few dry crumbs on it) then the cake is done. If it's got batter or wet clumps, wait a few minutes and try again. (Follow the link for more ways to test!)
As a rule of thumb I bake at about 320-325°F (160°C). This yields good results for me.
For larger items this is especially helpful as the items have more time to cook before burning or getting too much colour.
Josh is right about the toothpick test. It is the best way to determine if a cake is done.
One of the things to remember that a lot of times cooking time and temperatures will depend on your oven. Things like the temperature being calibrated properly or whether it is convection or not play a factor.