Gather all your ingredients and tools. Make sure your mixing bowl and beaters are clean and dry for the best results.
In your mixing bowl, pour in the heavy whipping cream. Using an electric mixer, beat the cream on medium-high speed until it starts to thicken, about 1-2 minutes.
Once the cream has thickened slightly, add the pure vanilla extract. Continue to beat the mixture until soft peaks form.
Sift in the unsweetened cocoa powder and powdered sugar. This step is crucial to avoid lumps and ensure a smooth mousse.
Using a spatula, gently fold the cocoa and sugar into the whipped cream until fully incorporated. Be careful not to deflate the mousse.
Spoon the mousse into your chosen serving dishes. You can enjoy it immediately or chill it in the fridge for a firmer texture.