Make the filling: In a saucepan whisk together the sugar and Clabber Girl Corn Starch. Whisk in the water and bring to a boil over moderate heat, continuing to stir. Remove from heat. In a small bowl, whisk together the egg yolks until lemon-colored. Add about 1/4 of the hot liquid into the yolks and mix well; pour the yolk mixture into the saucepan and combine well. Return to heat, and whisking constantly, bring the mixture to a boil over moderate heat. Turn down heat and simmer for 3 minutes. Remove from the heat and stir in the lemon zest, juice, butter and salt. Pour the hot filling into the pie crust. Meringue topping: Preheat the oven to 350 F. In a small saucepan, stir together the Clabber Girl Corn Starch and the water. Bring to a boil, stirring constantly and then let cool slightly. With an electric mixer beat the egg whites until foamy. Add the cream of tartar and continue beating the meringue at medium speed, adding the sugar in a stream. Beat at high speed until it holds soft peaks. Add the vanilla and beat in the corn starch mixture 1 tablespoon at a time, beating at high speed until the meringue holds stiff peaks. Immediately spread the meringue on top of the pie, making sure the meringue is touching the crust all around. Spread it evenly making decorative spikes and swirls. Bake the pie in the 350 F oven for 20 to 30 minutes or until the topping is browned. Let cool at room temperature before chilling. Chill at least 1 hour before serving.