I have seen it a million times and never noticed it. Only thing I can think of is that they were doing the scene a few times, and she got bored having nothing to do in that shot and started eating. Or maybe they were told it was just a run through and since she was doing nothing, she started eating the props thinking the camera wasn't running, but it was. Some directors have filmed what they are telling the actors is a run through just to get the raw emotions of the actor and not have the actor kind of clam up during a real take.
Here is another thing I think is Kim Bassinger in another Batman movie. Watch Batman And Robin (I know I know...it sucks) but when you during that stupid auction thing when Freeze crashes the party. The scene when Freeze needs the vine to swing up to Poison Ivy and he says "Boo" and the model drops the vine, that looks exactly like Kim. Even has the same little scream she did in Batman 89. I checked the credits and there is another name, but it looks and sounds like Kim doing a cameo. I could be wrong, but I swear that is her.