What, did you say floating chocolate. I'm going to tell you how to make a piece of chocolate float. Let's get started.

Step 1:

First you are going to take your glass,plastic etc. cup. Then you are going to fill it with 15 teaspoons or 1/4 cup plus 3 teaspoons of distilled white vinegar.

Step 2:

You are next going to add in your glass one square/ rectangle of Hershey's chocolate. That square of chocolate should clunk down to the bottom of you cup

Step 3:

Then you will add to your mixture one teaspoon of baking soda. That will bubble up and hopefully not overflow. But if it does overflow don't worry.

Step 4: Conclusion

You now should see you chocolate piece rise to the top of the mixture and float.

Just my guess:
It might be because of the bubbles that want to rise that push up on the chocolate causing it to rise. Or it might be something else. If you have a different conclusion please post in the comments