Round 15inch chocolate cake to feed 30/40!

[TheChosenTwo]

Please help! My neighbour is having a birthday party (I’m not invited - it’s not happening next door 😂) but has asked me to bake her a cake with the above description.
I do make cakes (not for a living, just for a hobby) but where do I start with a cake that’s this big? None of the recipes I use are for these quantities and what happens to the cooking time if I just double/treble the mix? Surely that just makes the edges extra dry?
Any help welcome!

Would she accept a rectangular cake instead of a round one, and maybe 2 cakes instead of one? (you could do 2 different flavours fairly easily, one vanilla and one chocolate). that would make the baking much easier.

I already have the large round tins and figured it would be relatively easy to just stack one on top of the other!
I’m not an experienced vegan baker so decided to keep it quite simple and she likes chocolate.
As I say, I just bake cakes for home and the occasional family birthday - they’re usually tasty although I don’t do all the decorative stuff.

Sorry, ‘hobby baker’ is actually quite a well used term for people that regularly bake for friends/family etc, I’m not really a hobby baker, just someone that makes cakes at home mainly for us to eat!!
And when I said my neighbour asked, I pretty much offered so she’s not being cheeky!