Combine sugar, flour, and salt in a heavy saucepan. Mix well to remove lumps. Gradually add milk, stirring until well blended. Bring to a boil; reduce heat, and cook, stirring constantly, until smooth and thickened. Remove from heat.
Beat egg yolks until thick and lemon colored. Gradually stir about half of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Cook over low heat 2 minutes, stirring constantly. Remove from heat; stir in vanilla. Cool to room temperature.
Place powdered sugar and peanut butter in a mixing bowl; cut in peanut butter with a pastry blender until mixture is crumbly.
Place half of peanut butter mixture in pastry shell. Pour cooled filling over mixture; top with remaining peanut butter mixture.
Beat egg whites (at room temperature) until foamy. Gradually add 1/2 cup plus 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves. Spread meringue over filling, sealing to edge of pastry. Bake at 375° for 8 minutes or until meringue is lightly browned. Cool to room temperature before serving.