Chocolate Starfish is slang for the rectum.
A chocolate starfish refers to the creased lines of skin from the anal area resembling a starfish and the chocolate bit is a reference to the brown colouring of that area – unless you have had it bleached!
Getting one’s anus bleached makes for a delightful passage (no pun intended) in the film “Bruno” by Sasha Baron Cohen.
It is often used to describe or speculate about anal sex, as in “taking it up the chocolate starfish”
“I bet he takes it up the chocolate starfish!”