Never having spawned before, I can't give you a personal guarentee about egg yolk, but I do know that a lot of breeders in Thailand use it; it's very high in protein, and I can certainly tell you that the grown bettas are enjoying it!
At Christmas, everybody comes home with me! That is why I am getting jars with lids, lol! They have to be moved anyway because Christmas is when I move out of the dorms into the new residence, so I'd have a huge upheaval anyway. They'll be over two months old by then, and hopefully they'll make the trip just fine.