In a large bowl with a handheld mixer or the bowl of a stand mixer fitted with the whisk attachment, whip the heavy cream on high speed until thick and frothy.
Stop the mixer and sift in the cocoa powder and powdered sugar. Add the vanilla, then beat again on low until cocoa powder and powdered sugar are mostly incorporated. You may also opt to fold the mixture with a spatula instead of whipping on low.
Increase the mixer speed to high again and whip until medium peaks form. Use immediately or chill until ready to use. Portion into dessert cups and add toppings (I suggest homemade whipped cream and/or berries) before serving. See notes2 for freezing instructions.
Dutch processed cocoa powder: this is preferred, but unsweetened cocoa powder will do just fine.
Freezing: freezing and thawing leftover chocolate mousse will diminish some of the “fluff” factor. Thaw in the refrigerator and rewhip in increments of 15 seconds to desired consistency.