Brownie Walnut Pie; 1/2 cup butter 3 ounces unsweetened chocolate, cut up 1 Pastry for Single-Crust Pie 3 eggs, lightly beaten 1 1/2 cups sugar 1/2 cup all-purpose flour 1 teaspoon vanilla 1 cup chopped walnuts 1 Hot Fudge Sauce or fresh fruit, such as raspberries, sliced strawberries, or sliced peaches, or vanilla ice cream (optional)