Stir chocolate and butter in medium saucepan over low heat until smooth. Whisk in next 4 ingredients. (Can be made 1 day ahead. Cover and chill.) Stir sauce over low heat until just warm. Whisk cream and powdered sugar in medium bowl to peaks; chill until ready to use. Arrange 2 banana halves in each of 4 long shallow dishes. Add 3 scoops ice cream to each dish. Spoon warm sauce over ice cream. Top with pineapple, whipped cream, and toasted coconut.