Place cocoa mix in a 3-1/2- or 4-quart slow cooker. Carefully stir in hot water and chocolate-flavored syrup.
Cover and cook on low-heat setting for 3 to 4 hours or on high-heat setting for 1-1/2 to 2 hours. Whisk in peanut butter and vanilla until smooth. Ladle cocoa into mugs. Have whipped cream available if desired. Whisk or stir cocoa occasionally.