Place a large Pyrex or stainless-steel bowl (or the bowl and beaters of your stand mixer) in the refrigerator or freezer and chill for 15 minutes.

Remove the chilled bowl and add 1 cup heavy whipping cream and ½ teaspoon pure vanilla extract.

Using an electric mixer on medium speed, beat the cream and vanilla for about 2 minutes until the mixture begins to thicken.

Add 3 tablespoons unsweetened cocoa powder (sifted) and ⅓ cup powdered sugar (sifted). Scrape the sides of the bowl with a spatula.

Continue whipping on medium speed for about 3 more minutes, or until the mixture is fluffy and soft peaks start to form; stop as soon as soft peaks hold to avoid overwhipping.

Cover and chill the mousse in the refrigerator for up to 3 days before serving.