In a food processor bowl, combine the 1-3/4 cups pecans and the 2/3 cup sugar. Cover and process until the pecans are very finely chopped. With the motor of the food processor running, add the 2 tablespoons melted butter in a steady stream. Process until the mixture is well combined. (Or blend pecans and sugar in batches in a blender container or chop pecans and combine with sugar. Transfer to a bowl and stir in the 2 tablespoons melted butter.)
Turn the pecan mixture into a 9- or 10-inch tart pan with removable bottom or a 9- or 10-inch springform pan. Press pecan mixture evenly on the bottom and about 3/4 inch up the sides of the pan.
Bake in a 350 degree F oven for 15 to 20 minutes or until crust is lightly browned on edges. Cool on a wire rack.
Bring the 1 cup cream just to boiling. Pour over chocolate in a bowl. Stir until chocolate is melted and mixture is well combined. If necessary, beat smooth with a wire whisk. Pour into the cooled crust. Cover and chill for 3 to 24 hours.
In a small heavy saucepan, melt the 3 tablespoons butter. Add the 1/2 cup sugar and the water. Cook over medium heat for about 4 to 5 minutes or until mixture is golden brown, stirring constantly with a wooden spoon. (Mixture may appear to separate, but will become smooth later.)
Remove saucepan from heat. Let cool for 5 minutes. Add 1/2 cup whipping cream (mixture may clump). Return saucepan to heat. Cook and stir over medium heat for about 3 to 5 minutes or until caramel mixture melts, and mixture becomes smooth (should have about 1 cup). Cover and cool. Chill sauce.
Remove sides of tart pan. Spoon sauce over top. Sprinkle with remaining 1/2 cup pecans. Makes 12 to 16 servings.