Preheat oven to 375 degrees F (190 degrees C). In a microwave-safe bowl, microwave chocolate chips until melted. Stir occasionally until chocolate is smooth.
Cream butter or margarine with sugar. Beat in eggs, one at a time. Add melted chocolate, instant coffee, and rum extract. Stir in flour and broken walnuts. Turn filling into pie shell. Top with walnut halves in decorative pattern.
Bake at 375 degrees F (190 degrees C) for 25 minutes. Cool.