Place egg yolks in top of double boiler; add sugar and salt. Gradually add boiling water, stirring constantly. Add vinegar and cornstarch dissolved in cold water. Cook over boiling water until thick and smooth, about 12 minutes.
Remove from heat. Add lemon extract. Stir until filling is smooth and blended, scraping sides of pan.
Pour hot filling into pie shell. Top lukewarm filling with meringue (see Perfect Meringue for Topping Pies), spreading to edges and sealing to crust. Bake in moderate oven (350°F.)12 to 15 minutes, or until meringue is lightly browned.