It has a bright yellow exterior, and, according to Patrick, has a delicious creamy filling inside, which is seen in "InSPONGEiac."
In SpongeBob vs. The Big One: Beach Party Cook-Off, they appear to be part of SpongeBob with green frosting on them.
Role in series
A sponge cake first appears in the episode "To Save a Squirrel" when Patrick deems himself so hungry that he could eat one. SpongeBob runs away in horror upon the syllable pause in between the words "sponge" and "cake," after which it turns out that Patrick just wanted to share a sponge cake with him.
It later appears in "InSPONGEiac" when Patrick thinks SpongeBob is one and bites his hand.