Welcome to the Cake Decorators Q&A

Hello,
I’ve used your ready-reckoner to calculate that a recipe I use with a 9″ round tin would need to be tripled for a 15″ square. So far, so good but my query is about increasing the baking time accordingly.
The cake in question is a lemon and sour cream sponge where one quantity of the recipe would normally need to cook at 150 – 160 C for 2 hours and produces , when properly cooked, a very thin sugary crust on top and a moist, dense sponge which I think would be great for carving.
I was already going to increase the cooking time to three hours before I used your ready-reckoner but now that I realise I’m going to have to triple the quantity of batter, I’m wondering if I would need to increase it further. Any suggestions? Thanks, Delphine

Hello Delphine

Firstly just to point out that the ready reckoner doesn’t actually go up to 15″ conversion. It goes to 14″ which would increase your recipe 3.1
For 15″ square tin the recipe would need to be increased by 3.5 You may find using the cakeometre better for converting your recipe. You’ll find it on the same page as the ready reckoner.
I have a very similar recipe which uses buttermilk instead of soured cream. I wouldn’t take my recipe up to 15″ cake because it is extreamly moist and apt to collapse. My instinct would be to bake in two halves, keeping an eye on the timing rather than depend on given times which are only guidelines. I unmould my recipe when completly cold, it breaks if even slightly warm.
You haven’t said what type of flour you are using, the sugar to flour ratio and whether there is extra leavening required. In my experience large cakes which require additional leavening bake better at lower temperature for longer to prevent collapse. A square cake especially benefits from double lining the tin, using cake strips and heating core nails to help distribute the heat in the centre of the batter. The corners which bake a lot faster and burn should be be protected as much as possible. I usually make more strips and line each corner three or four times before lining the sides. Stick the parchment down with a little butter so it doesn’t fold back on itself.
I’m also assuming that your oven is large enough to fit a 15″ cake pan with sufficient space all round for even heat distribution and more vital to close the oven door! I’m sure you know all this already, I just wouldn’t be confident enough to bake such a large cake in my domestic oven, assuming that yours is.
I hope other members will chime in. This is just my own opinion and not necessarily suited for all. Wish you luck with your cake!