The word "engineer" can be very general, and you think of it kind of like the word "scientist". While "scientist" describes a person who seeks out new knowledge, that knowledge could be about just about anything. So you would imagine that a "chocolate scientist" would study chocolate. Engineers are people that generally use science and math to create something new, so you can think of a "chocolate engineer" as someone who works on creating innovations in chocolate. It's not really any more specific than that. To learn more you might want to read this post about engineers who make chocolate.