In Toni Morrison's Jazz, Dorcas Manfred is an 18-year-old black girl with whom Joe falls in love. She is described as "the cream at the top of the milk" because she has a light complexion. Moreover, Joe is married to Violet and obviously he should not be having an affair with another woman--particularly one who is just a girl--yet Joe finds Dorcas irresistable. Dorcas is highly motivated to explore her sexuality, so she enters the affair with Joe to see what it will bring. Joe is happy to be with Dorcas because she is young and beautiful, and he goes to all ends to hide the affair and keep it going. He is so taken by Dorcas that a jealous rage causes him to shoot her one night at a party. So Morrison's description of Dorcas is intended to portray her and beautiful and desirable.
Because she is a delicacy, the best part of her kind. Also please note that Dorcas is described as light complected and having hair that doesn't need pressing. Cream at the top of unprocessed milk is slightly yellow.