Yes you can order it to go. Just ask either the hostess at the reception desk or the bartender at the lounge.
I have been turned down a few times when special/private events or unusual hours have severely restricted the available menu items. On one such occasion, the manager recommended I try begging room service to send a slice to my room. Too bad I wasn't staying at the
Disneyland Hotel at the time! One time the bartender said they weren't serving dinner at the lounge. But the hostess always still able to get one for me. Sometimes it just takes a smile and a friendly request, and you're in chocolate cake Heaven.