First, can you find some "gift" that she *can* give you at "traditional chocolate-giving time"? Like, jelly beans, or Peeps, or pistachios, or a scarf, or something?
Then you can redirect her, which may be more powerful than simply saying "don't" (it's always better to focus on what you want rather than what you don't want).
And then you can do the "face light up, I'm so excited" thing to reinforce it. Since she wants so badly to please you, that could be very powerful.
Also, if she does give you chocolate, give her negative reinforcement that doesn't make her defensive.
Cry, quietly, and then apologize for crying. Have DH comfort you.
but don't get angry, bcs that puts her on the defensive, which means she locks herself into her "I give chocolate" rhetoric.
The other option is to drop the rope.