Line a 9-inch pie plate with pie crust, fold edges under, and flute as desired. Place in refrigerator to chill while you work on filling.
In a medium saucepan, heat the 1/2 cup water to a boil. Turn off heat, then add chocolate and stir until melted. Add the 2/3 cup sugar and bring to a boil once more. Once boiling, remove from heat and stir in the 1/4 cup butter and 2 teaspoons vanilla. Stir and set aside.
In a medium bowl, sift together the flour, baking powder, and salt for the cake layer. Set aside.
In a separate large bowl, cream together the butter and sugar with an electric mixer for one minute. Add egg and mix until combined. Alternate adding milk and flour mixture, then add the vanilla and mix until combined.
Transfer batter to chilled pie crust, then smooth out the top with a rubber spatula.
Stir melted chocolate mixture, then pour over cake batter layer.
Sprinkle nuts over the top, then bake until a toothpick inserted into the center comes out clean, about 45 minutes.
Let cool, then slice, and serve with whipped cream, if desired. Enjoy!
