1a would not be used, all the others seem fine. But of course, why not? ((And now I've just seen that Ootmaster says it's okay.))
I'm trying to think to myself what it is about cake/pizza that makes them different--why 2a is fine.
Some things seem to have a mass noun (uncountable) aspect to them, even tho they can still be counted.
My mother is baking bread. ~a loaf of bread. ~some bread.
But if you say ~some breads, then the meaning changes, i.e., she is making several kinds of bread.
You can say:
My mother is making brownies. ~some brownies.
But not: ~making brownie. ~making a brownie.
( ~a really big brownie, < sounds okay)
Brownies are made like cakes (in a baking pan, like many cakes), you call them brownies all along the way, even before they've been cut up (like cake). In this way, they're like cookies ( ~cookies, ~some cookies, but not ~cookie, or ~a cookie)
Then, you ask: would you like a piece of cake/pizza?
But: would you like a brownie.
Bread and pizza seem to share something here, conceptually (tho not completely)..., is it they use leavened doughs?
Pizzas are countable as 1 pizza, 2 pizzas..., and cakes are counted the same way; and for both people ask: would you like a piece/slice of ~
For bread you count 1 loaf, 2 loaves..., and still offer a slice or piece.
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Sorry I can't help. And lucky it's not earlier in the day, or I'd be hungry.