I'd skip the chart because to make it a reward says that baby talk is a highly valued activity. And, making her act grown up in order to earn the baby time may reinforce the ambivalence she's already feeling about being big. If what she's looking for is that feeling of complete indulgence babies get, if make her earn it or only give in begrudgingly she may not really get that feelings she's looking for. In other words, I think it could backfire and increase the behavior. So, instead I'd treat baby talk as a play activity you will fully indulge as a separate activity from normal conversation. "Oh yes, I'd like to play baby with you just as soon as we are finished with lunch, but right now I need to hear your regular voice so I can understand you."
If it is more that she's whining, I would role play the difference between a whiny voice and a regular voice. Practice together and have fun doing exaggerated versions. Then in the moment with the nuggets you are in your rights to ask her to "try again in a regular voice."
