In a medium bowl, combine sugar, flour, and salt. Add softened butter. Mix beaten egg, milk, and vanilla. Add to bowl. Mix for 20 seconds.

Transfer to a medium saucepan and bring to a boil; reduce heat to barely a simmer and cook, uncovered, until mixture has thickened and will coat the back of a spoon. Add chocolate that has been warmed until liquid in the microwave. Stir until melted.

Pour into pie shell. Cover with plastic wrap so that a skin will not form and refrigerate for at least 90 minutes.

Serve with whipped cream or Cool Whip.