1. Combine first 4 ingredients in the top of a double boiler. Cook over simmering water until chocolates completely melt, stirring constantly. Pour mixture into a 9-inch pie plate. Cover surface with plastic wrap. Refrigerate 2 hours or until firm.
2. Scoop chocolate mixture with a spoon into 15 equal portions. Roll each portion into a ball. Place cocoa in a shallow bowl. Roll chocolate balls in cocoa to coat completely. Refrigerate until ready to serve.