Put a stand mixer bowl and its whisk attachment (or, if you’re using a hand mixer, a large stainless-steel bowl and a whisk) in the freezer to chill for about 10 minutes.
While the bowl and whisk are chilling, stir the water and the gelatin together in a metal measuring cup or a very small saucepan and set aside for 5 minutes. Then, dissolve the gelatin over low heat, about 2 minutes. Remove from the heat.
Retrieve the mixing bowl from the chill and pour in the cream, cocoa mix, and vanilla. Mix on low speed until thoroughly mixed, about 2 minutes.
With the mixer running, drizzle in the dissolved gelatin solution, boost the speed to high, and whip until medium peaks develop, 1 to 2 minutes. Frost cake as desired.