Combine. In the bowl of a stand mixer (or a large glass or metal bowl), combine cream, powdered sugar, cocoa powder, and vanilla.
Mix, Then Whip. With the whisk attachment (or a hand mixer), beat on high speed for 30-60 seconds, then scrape down the sides of the bowl. Mix again on high speed for 3-5 minutes until you reach stiff peaks. Watch carefully when you can see the trail of the beaters in the whipped cream. Check it and mix just until the whipped cream holds its shape on the end of the beater.
Serve. Keep the whipped cream cold and covered until you're ready to use it (it'll set up even more when chilled!). Serve as a fruit dip, pipe onto brownies, cakes, pies, cheesecake, etc.
Store. Keep whipped cream in an airtight container in the fridge up to 3-4 days. Since this is a stabilized whipped cream, it should hold its shape well, but since it's whipped cream, it'll also need to stay cold (it'll melt in warm temperatures)! Keep refrigerated when not in use, and don't keep out at room temperature longer than 1 hour (less, if it's warm!).