You don't need two three tier cakes on the same day. That's ridiculous. Surely you only need one tier altogether anyway?
If you are going to make three, could you not make two of them for your boss, and one for her? Or do four altogether, but make hers separate, but part of the whole (somehow). Then everyone, front and back staff, get together and do birthday celebrations for both at the same time.
I do understand why you've kept it to your own team up til now, and this is setting a precedent for other staff (which, I agree with pp, the business should cough up for and let you, or another chef, bake for them) but it touches a nerve. First workplace I was at one woman would always bake a cake for our birthdays. The last year I was there I was bullied, badly, by the lot of them. There was no way I was going to be there on my birthday (didn't usually worry me) and they knew I wasn't going to be there. Still, she baked me a cake on that day, they all had it, and then told me the next day how nice my cake was. Charming!
So please, try and include her somehow, though don't pander to her OTT demands.