Perhaps I am overreacting, but in all this discussion I started wondering who was taking care of this question on the other end. And while I love the idea of a freshly baked, store-bought cake for use in this show, I think this needs a step back.
Perhaps I misunderstood who is responsible for this. Seems to me this is a props issue, w input from Stage management about any food issues. Are we suggesting the SM bakes the cake?
Ingredients: That's $1 or more for the mix plus eggs, milk, oil - and TIME (!!!!!) to purchase these things, mix and bake the cake, the sheet pans necessary to cook it in and transport from, and the schlepping and storing issue itself, all on top of our already long day. Yes, a mix is a great idea, if someone else is doing it and being reimbursed and all. But to plant in your producer/director/folks running the joint's heads that this is part of the SMs job is the foundation for a major problem later on. Suggestions are terrific. Collecting ideas for the props person is great - and finding solutions others have used is one of the great resources of this site. But actually doing this? It's not your job.