In a saucepot over high heat, combine sugar, cornstarch, flour, salt and water. Bring to a boil and stir until thickened, about 3 minutes. Combine the lemon zest, juice and extract with the yolks. Temper the yolks with the hot sugar mixture. Stir in the butter and strain through a fine mesh strainer if necessary. Put the mixture back over heat and cook for another 3 minutes. Transfer the custard to the pie crust, hot.
Preheat oven to 350 degrees.
Meringue: In a small saucepan over medium-high heat, stir together the sugar and water until the sugar is completely moistened. Cook until the syrup boils. Turn the heat to low. In the bowl of an electric mixer, beat the egg whites until foamy. Add the cream of tartar and beat until stiff peaks form. Bring the sugar syrup to 236 degrees and slowly drizzle the sugar syrup down the side of the mixing bowl into the whites with the motor of the mixer running. Beat the whites for an additional 2 minutes until shiny.
Top the custard with meringue, when the custard has cooled to room temperature. Seal the edges carefully, and transfer to the oven. Bake until browned, about 10 minutes. Refrigerate until ready to serve.