Combine sugar, flour, cocoa, and salt in a large heavy saucepan, stirring to remove lumps. Gradually add scalded milk, stirring until well blended. Cook over medium heat, stirring constantly, until mixture comes to a boil and thickens. Cook an additional minute, stirring constantly to prevent scorching.
Gradually stir one-fourth of hot mixture into beaten eggs; add to remaining hot mixture, stirring constantly. Cook, stirring constantly, 2 minutes or until mixture thickens. Remove from heat; add butter and vanilla, stirring until butter melts. Pour mixture into baked pastry shell; chill thoroughly. To serve, garnish with whipped cream and chocolate curls.