Combine brown sugar, cornstarch, and 1/4 teaspoon salt in top of a double boiler; add milk and syrup, stirring well. Cook over boiling water, stirring constantly, 20 minutes or until thickened.
Beat egg yolks until thick and lemon colored. Gradually stir one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Cook 5 minutes over boiling water, stirring frequently. Remove from heat; stir in butter, vanilla, and 3/4 cup pecans. Pour into pastry shell.
Beat egg whites (at room temperature), cream of tartar, and pinch of salt until foamy. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form. Spread meringue over filling, sealing to edge of pastry. Sprinkle with remaining 1/4 cup pecans. Bake at 350° for 12 minutes or until meringue is golden brown. Cool to room temperature. Chill thoroughly.