In a 3- to 4-quart slow cooker combine cocoa powder, sugar, and salt. Stir in the hot water until mixture forms a paste. Stir in peanut butter. Whisk in half-and-half and milk.
Cover and cook on low-heat setting for 5 to 6 hours or on high-heat setting for 2 1/2 to 3 hours. (Do not boil.) Add bittersweet chocolate, whisking until chocolate is melted.
If necessary, skim "skin" from surface; discard. Serve immediately or keep warm, covered, on warm-heat setting or low-heat setting for up to 2 hours.
About 30 minutes before serving, microwave hot fudge ice cream topping on 100 percent power (high) for 30 seconds; stir. Use a spoon to splatter the warm topping around the insides of glass mugs or thick-sided clear glasses; let topping set up.
Before serving, stir in vanilla. Carefully ladle hot cocoa mixture into prepared mugs or glasses.