Printing cookies sounds like way too much fun...
It does work though--Frog's right on--you stick it in a tub of water to stop the reaction, and you work fast, because as soon as you load "ink", you're reclaiming the screen.
And it's not white, as mentioned, it's more of a 'natural' look. Using gelatin or cornstarch is a lot cheaper than using the toilet crap--and you can make stuff that actually prints OK.
I suppose chocolate would be fun though too-- keep some cocoa and melted butter around to fine tune the viscosity...