It resembles a regular Krabby Patty, except it is gray and transparent.
Role in episode
It is created by SpongeBob when the pile of Krabby Patties becomes burnt on the grill. The smoke from the burnt patties takes on the shape of a Krabby Patty, thus creating the Night Patties.
When SpongeBob serves them to the customers, they love them, and praise him for the food.