Preheat oven to 375F. Microwave chocolate in microwaveable bowl on HIGH 2 min. or until almost melted, stirring after 1 min. Stir until chocolate is completely melted; set aside. Beat butter and sugar in large bowl with electric mixer on medium speed until light and fluffy.
Add eggs, one at a time, beating until well blended after each addition.
Add chocolate, coffee and rum extract; mix well. Stir in flour and chopped walnuts.
Pour into pastry shell. Arrange walnut halves on top of pie.
Bake in lower third of oven for 25 min. Cool. Refrigerate at least 1 hour before serving.