Chop the bittersweet chocolate into small bits. In a small pan, warm the cream and butter until it is almost boiling. At the same time, heat water in a double boiler. When the cream mixture is hot, put the chopped chocolate in the top of the double boiler, slowly pour the hot cream over the chocolate and whisk until the chocolate melts and is smooth. Remove from heat at the earliest possible moment, as leaving it on too long will cause the chocolate to become grainy.
Refrigerate for 2 hours. Scoop out mounds and roll into small balls. Into a doubled paper lunch sack pour the cocoa and all the truffle balls. Close the sack tightly with your fist, then turn it upside down several times to coat truffles with cocoa. Pour the truffles out gently into a large sieve, shake gently a few moments over a large bowl to catch the loose cocoa, then pour the truffles into a bowl. Put about 5 truffles at a time back into the sieve and shake them back and forth until there is no more loose cocoa on them or in the bottom of the sieve. This removes all loose cocoa plus it gives the truffles' surface a nice texture.
Package truffles in small boxes.