If your kitchen is warm, set a medium metal mixing bowl over a larger bowl filled with a handful of ice to keep everything cold. Keep the cream refrigerated until needed.
Sift the hot cocoa mix into a small bowl and stir in the pinch of salt to eliminate any lumps and evenly season the mix.
Make a cocoa base: add 2 tablespoons of the cold cream, plus the vanilla, to the cocoa mix and mash into a smooth, glossy paste with a spoon.
Pour the remaining cold cream into the chilled mixing bowl. Begin whipping on low speed, then increase to medium and beat until the cream shows soft lines and lightly mounds, 2 to 3 minutes.
Scrape in half of the cocoa paste and beat on medium-low just to combine. Add the rest and continue mixing, then raise to medium-high for 30 to 60 seconds until medium-stiff peaks hold. Stop as soon as the peaks stand with a slight curl—do not overbeat.
For clean scoops or piping, chill the whipped cream 5 to 10 minutes to set. Use immediately or refrigerate, covered, up to 24 hours.