Mix sugar and flour, add vinegar and beaten egg yolks. Mix well.
Add 1 cup hot water. Heat remaining 1 1/2 cups water to boiling and stir in egg yolk mixture. Boil mixture until thick. Remove from heat.
Add lemon extract and butter. Cool. Pour into a baked pie shell.
Preheat oven to 350 degrees F (175 degrees C). Beat egg whites until stiff; add 6 Tablespoons sugar and 1/4 teaspoon cream of tartar and continue beating until it holds a peak. Pile meringue onto filling. Bake until meringue is lightly browned, 10 - 15 minutes.