Melt the 1 pound of chocolate slowly in a double boiler. Cool slightly, then combine with the heavy cream. Stir in the butter and rum and refrigerate until firm but still pliable, about 20 minutes. Transfer to a pastry bag fitted with a large straight tip and pipe into teaspoon size mounds on a sheet of waxed paper, or, alternately, spoon out the teaspoon size mounds. Refrigerate until firm. Working quickly to prevent melting, roll the chocolate into balls, then chill again until firm. In a food processor fitted with a metal blade, pulse the toasted coconut briefly to make it a finer to help it adhere better to the truffles. When firm, roll each truffle in the coconut, pressing firmly to make sure the coconut sticks. At this point you can refrigerate the truffles for as much as a day, until you are ready to dip them.
Melt the dipping chocolate in a double boiler over hot, not boiling water. Using a wire dipping spoon, or a regular teaspoon and your fingers, dip each truffle briefly into the melted chocolate. Remove to a cake rack set over a cookie sheet until dry, and refrigerate until firm. You may scrape up any chocolate that has dripped to be returned to the dipping pot. The truffles will keep for several weeks if refrigerated in a covered container.