The Fashion Star mentor gave birth to her first child Maxwell Drew earlier this month and has asked her long-time friend who appeared on MTV show Newlyweds - to take on the role.
"CaCee and Jessica have been through so much together and Jessica loves her so much, it only made sense that she would ask her to be godmother," a source told In Touch magazine.
Simpson and her fiancé Eric Johnson are busy planning Maxwell's baptism but cannot decide whether to have it at their home in Beverly Hills or in Jessica's hometown of Waco, Texas.
Cobb is currently engaged to Scrubs actor Donald Faison.
Meanwhile Simpson and Johnson are also organising their wedding, which is due to take place in Hawaii on December 12.
An insider told Life & Style: "Jessica and Eric are eyeing the Hawaiian island of Kauai for the reception for around 60 family and friends."
The couple are said to have hired celebrity party planner Mindy Weiss for both celebrations.