Preheat oven to 375F. Bake pastry shell 10 minutes; remove from oven. Reduce oven temperature to 325F. In sausepcan over low heat, melt chips with butter or margarine. In mixer bowl, beat chocolate mixture with condensed milk, bisucit mix, eggs and vanilla until smooth. Add nuts. Pour into pastry shell. Bake 35-40 minutes or until center is set. Serve warm or at room temperature with vanilla ice cream. Refrigerate leftovers.