Combine corn starch and sugar, add water slowly, stirring constantly, until thick and smooth.   Add slightly beaten egg yolks, butter, lemon rind and juice, and salt.  Cook 2 or 3 minutes.  Pour into baked shell.  Cover with meringue made from 3 egg whites beaten stiff, and 2  1/2 Tbsp.  sugar.  Bake in slow oven 15 minutes, or until light brown.