Grease a shallow 11" x 7" baking pan with butter; line base with parchment or waxed paper. Heat white chocolate, butter, and half of rum together in a large bowl placed over a pan of gently simmering water, stirring until well blended. Remove from heat; stir in remaining rum. Roughly chop pistachio nuts; add 3/4 of nuts to chocolate mixture with cherries and coconut. Mix well. Spread mixture evenly in prepared pan. Finely chop remaining nuts. Put melted semisweet chocolate in a small paper piping bag; cut a small hole in the bottom of bag. Pipe squiggly lines of chocolate all over top of mixture, then sprinkle with finely chopped nuts. Chill 4 to 5 hours or until firmly set. Cut in 30 pieces. Remove from pan with a flexible spatula. Keep refrigerated or in a cool place until serving. From "The Book of Cookies" by Pat Alburey.