Preheat oven to 350°F. Spray a 9-inch pie plate with nonstick spray.
Combine sugar, butter and water in medium saucepan. Cook over medium-heat, stirring occasionally, just until mixture comes to a boil. Remove from heat. Immediately add chocolate chips; stir until melted. Remove from heat and let cool for a few minutes. Add eggs; beat with spoon until well-blended.
Stir together flour, baking soda and salt. Add to chocolate mixture; stir until well-blended. Stir in vanilla and nuts, if desired; pour into prepared pie plate.
Bake 25 to 30 minutes or until almost set. (pie will not test done in center) I like to pull it out a little early since I like gooey brownies!
PREPARE THE FUDGE SAUCE:
Combine all sauce ingredients in medium microwave-safe bowl. Microwave on HIGH 1 minute; stir. If necessary, microwave on HIGH an additional 15 seconds at a time, stirring after each heating, just until chips are melted and mixture is smooth.
Top warm pie slices with 1/3 cup scoops of ice cream and a drizzle of the fudge sauce.
shortcut tip: purchase hot fudge from the store instead of making this homemade version.