JJ75, You do need to be careful about the cooking times as it is possible, with a larger cake, that you will get the outside decidedly crisp before the inside is done. We tried baking 12" madeira cakes here at work and baked four of them over about a month, with various tweaks to the method and didn't quite manage it. Someone else did though, and I have copied her method below (she was happy for us to pass it on, as I have on MN before).
Madeira Cake - 12” Square (with thanks to Jen Aldred)
(Cooking time 65 mins)
Ingredients:
825 gm Butter
825 gm Self Raising Flour
825 gm Caster Sugar
12 Large Eggs
Method:
Pre-heat the oven to 180 deg. (Gas Mark 4)
Grease and line the pan (Triple lining is suggested to prevent scorching).
Take a LARGE mixing bowl, cream the butter and sugar. Then add the eggs, one at a time to prevent curdling (you may need to add a little flour from time to time). Once the eggs are mixed in, fold in the balance of the flour.
Cook for 30 minutes at 180 deg. then cover with baking parchment for another 20 minutes at the same temperature. For the final 15 minutes, keep covered but reduce the temperature to 170 deg. (Gas 3).
This should produce a cake that has a close, even texture, is completely cooked and moist.
To adapt for chocolate cake: (with thanks to Lorna Shannon)
Substitute 3 desert spoons of good cocoa powder for 3 desert spoons of flour. Then add a further 2 heaped desert spoons of cocoa powder.
You can substitute 200gm of the butter for Flora to make it lighter. This should result in a bouncy, good textured chocolate sponge. Finished off with a dark chocolate ganache, sprinkled with white chocolate shavings, it is quite a treat!
I hope that this is helpful.