Grind pecans in food processor until fine. Melt butter in saucepan. Add sugar and heat, stirring with wire whisk, to thick smooth paste. Remove from heat and add water, stirring constantly. Pour syrup over pecans and mix thoroughly. Let cool slightly. Butter 12-inch tart pan. Press pecan mixture over bottom and up sides of pan. Bake at 325? 10 minutes or until golden brown. Remove and let cool at room temperature. Melt chocolate in top of double boiler. Stir in oil until mixture is smooth and almost liquid. Whip cream until soft peaks form. Add hot chocolate, folding gently until well blended. Refrigerate 2 hours. Remove mousse from refrigerator and stir to make sure mixture is smooth. Spoon into pastry bag and pipe into pecan crust, or spoon evenly into crust. Refrigerate. Before serving, decorate with whipped cream and whole pecans, if desired. To serve, spoon Toffee Sauce onto bottom of dessert plate and top with tart, or serve sauce separately, Makes 16 to 20 servings.