Forgot to add cornstarch to my dough which is already proofing

If the dough is proofing correctly, which it should be if the cornstarch is the only thing missing and everything else is correct, then you should be fine without it.

Depending on how much cornstarch was asked for the texture will be different, probably a tad less tender. But it shouldn't be anything that ruins the dough.

If you can get sweet dough like that through its first proof with good rise it's pretty much smooth sailing after that so I wouldn't worry about it.

If you want you can post the recipe and I'll take a gander at it.