In a saucepan, bring the cream just to a simmer over low heat. Pour the cream over the chocolate in a bowl and let stand about 10 minutes to melt the chocolate. Add the vanilla and stir until smooth. Set aside to cool for 1 hour at room temperature. Then beat the chocolate at medium speed until it gets thick and light colored. Spread over the bottom of a baking dish and smooth the top. Refrigerate about 2 hours, until firm.
Pour the cocoa powder onto a deep plate or shallow bowl. Use a melon baller or small ice cream scoop to scoop out balls of chocolate; place them on the plate with the cocoa powder and roll between 2 forks to completely coat. Then use the forks to carefully transfer them to a parchment or waxed paper lined baking sheet.