Begin by chilling your mixing bowl in the refrigerator for 15-20 minutes. If you forget, it's fine, but the chilled bowl does seem to produce slightly better results.

Add heavy cream, powdered sugar, peppermint extract, and cocoa powder to the bowl of an electric mixer that's fitted with the whisk attachment and mix on high speed, scraping the sides down in the beginning, until cream forms stiff peaks and holds it shape. Keep in the refrigerator until ready to serve.