Chill bowl and beaters for 15 minutes before beginning.
Add heavy whipping cream to the bowl.
Beat until just beginning to thicken. You should be able to see tracks in the cream.
Add the cocoa powder and powdered sugar.
Continue beating until whipped cream reaches desired consistency. Do not overwhip or it will begin to turn to butter.
Chill any leftover whipped cream.
Notes
Whipping cream: It's important that the cream be cold so it will whip up better.
Cocoa powder: I use regular unsweetened cocoa powder or Hershey's Special Dark cocoa powder in this recipe. I haven't tried using Dutch-processed cocoa powder in this recipe.
Powdered sugar: Please see the notes above the recipe for using granulated sugar.
Flavorings: I also have notes above about adding flavorings to the whipped cream.