First, a few quick tips from me. Then a link.

Wash your hands. You might think they are clean, but there is still a bit of chocolate smell lingering...

Never feed food or treats through the bars of their cage. Feeding them this way teaches them they need to grab whatever comes through as it might be food. Always open the door and put your hand inside while holding the treat.

"Eeep!" in rat language communicates that you are being groomed too forcefully. When rat teeth are causing pain or blood, "Eeep!" is the best way to communicate they should stop. Hitting your rat or smacking its nose communicates your hand is to be feared and can lead to fear biting which will certainly cause blood.

Not all nipping is bad. Here is a lot more information on the subject.