SPOILERS WARNING
The baker needs chocolate. You can find chocolate in the fey wilds.
The chocolate melts normally. You need to boost the appetite of the chocolate so that it doesn't melt. Appetite comes from royalty.
The actual act of getting the chocolate into the party. I might be misremembering the method and just doing it wrong, but whenever I find a way to feed the chocolate royalty they still just melt. My method is eating the fey royalty as a male character and then using that as a workaround to feed them to a chocolate with the fuck action. I seem to remember getting a specific interaction out of this that added the chocolate to my party the first time I tried it.
Edit: I'm pretty sure it's because of a lack of feed stat after some more experimentation in a cheat save. Adding achievement points to the feed stat now and going to try and do it in a regular save.