Fudge surprise key lime pie

1 cup granulated sugar
2 tablespoons cornstarch
1 envelope unflavored gelatin
2 beaten egg yolks
1-1/4 cups milk
2 tablespoons butter, cut up
1/3 cup lime juice
1/2 cup fudge ice cream topping
Baked 9-inch pastry shell, cooled
1/2 cup finely chopped macadamia nuts
1 8-ounce carton dairy sour cream
Green food coloring (optional)
1/3 to 2/3 cup whipping cream
1 tablespoon powdered sugar
Coarsely chopped macadamia nuts (optional)
Chocolate shavings (optional)

1. For filling, in a medium saucepan, combine the granulated sugar, cornstarch, and gelatin. Combine egg yolks and milk; add to sugar mixture in saucepan. Cook and stir over medium heat until thickened and bubbly; reduce heat. Cook and stir for 2 minutes more. Remove from heat. Add butter, stirring gently until melted. Gradually stir in lime juice. Transfer to a bowl; cover surface with plastic wrap. Set aside to cool slightly (30 minutes).
2. Meanwhile, heat fudge topping just until warm. Spread over bottom of pastry shell. Sprinkle with the 1/2 cup finely chopped nuts. Chill until firm.
3. When filling has cooled, gently stir in sour cream. If desired, add 1 or 2 drops of green food coloring. Chill until mixture mounds (45 minutes to 1 hour). Spread over fudge layer. Cover and chill pie for 2 to 24 hours.
4. To serve, beat whipping cream and powdered sugar with an electric mixer on high speed until soft peaks form. Spread whipped cream over center of pie. If desired, sprinkle with chopped nuts and chocolate shavings.