Android? We should have libraries for voice command (probably around android.speech ). I checked once because I would like to be able to voice control a GPS and mapping system while driving. If you wanted to voice control the application, that would be surely much more challenging, yet possibly interesting and exportable to other future needs.
This given, for the specific needs as you presented them I would have taken an old tablet, coded the feats you needed (timer, converter etc) in a single interface, fixed it in some most appropriate part of the kitchen for that use, and covered it with some plastic wrap film (I think standard PVC should be capacitive display friendly) in order to keep electronics and mess separate.
Of course, if you are more comfortable with the assistant, that is anyone's prerogative. Just reasoning.