Whisk cocoa powder and icing sugar in heatproof bowl; set aside.
Heat cream in small saucepan over medium heat, 2 to 3 min., until steaming.
Pour over cocoa mixture. Whisk until smooth. Stir in vanilla and salt. Cool to room temperature. Cover and keep refrigerated up to 1 week. Serve as a dip for fruit or over ice cream sundaes.