I realize this may be an impossible question with all the varieties out there, but any idea what variety of epiphyllum this is?

A size might help. Med, large, x Large? There are several sites that list them, and most mention the flower size. My only yellow is medium. Here is a good one
http://www.epigalleria.com/ind...

I don't know how to determine size. All of my epis are about the same size, so I'd say that whatever is typical is what it is. Here is a picture of the whole plant. The pot is around 14 inches in diameter.

It reminds me of my Hybrid Epiphyllum (Epiphyllum 'Fruhlingsgold')

Wow, Kelli ... that is one awesome plant!

I looked at the web site and of those listed there, it looks the most like 'Princess Kelly S.' Description of the plant fits, too (thick, flat).

If you looked at the site, now you know the sizes. There are so many hybrids out there that you'll probably never know the name for sure. It seems each grower gives it a different name. I looked through my pics and yours looks just like my 'Good as Gold', but as I said, that is a medium flower.