Whip the heavy whipping cream with a handheld electric mixer until stiff peaks form, but don't over beat. Pour in the sweetened condensed milk, cocoa powder, and vanilla. Mix briefly on low speed just til combined well. Pour into a large bread loaf pan or other freezer safe container, freeze 4-6 hours, or overnight. When you pull it out from the freezer, let it sit just a few minutes to thaw slightly for easier scooping.