Crust: In a food processor combine pecans and sugar until very fine. Combine with melted butter and press up sides on bottom of lightly greased pie pan. Chill in freezer 15 minutes then bake at 350 for 10 minutes. Cool. Filling: Chop chocolate to size of a cough drop. In sauce pot, heat cream to boiling and off heat, add chocolate to cream. Stir until smooth. Add nuts and pour into cooled pie shell. Chill until firm, about 1 hour. Caramel topping: In heavy 2-quart pot, melt butter, add sugar and stir as it comes to a medium boil. Boil, stirring gently until mixture becomes a nutty brown. Remove from heat and slowly and carefully add cream. Mixture will sputter. Stir gradually until cream is incorporated. Strain this through a sieve and cool to room temperature (placing bowl in ice water will speed the process), then pour caramel sauce over cold tart and garnish with pecans. Serve with whipped cream if desired.