Combine cocoa, sugar, and 2 cups heavy cream in the bowl of a standing mixer, or a large bowl (if using an electric hand mixer). Mix on high until light and airy, at least 5 minutes. Transfer to small dishes or jars, cover, and refrigerate at least 1 hour until completely chilled, light, and fluffy.

When ready to serve, top mousse with sliced strawberries and mint leaf. Feel free to add whipped cream if desired.