Preheat the oven to 375 degrees.
Place the crust in a 9 inch pie pan.
Line the inside of crust with aluminum foil (don’t turn it down over the rim, but leave the extra sticking up so that you have something to hold on to). Fill the foil all the way up to the top of the shell with pie weights or dried beans. Bake 25 minutes, until dry and beginning to turn “blond.” Lift the foil and weights out of the shell and bake another 10 to 15 minutes, checking frequently to prevent over baking, until medium brown.
Lower oven temp to 350 degrees.
In a sauce pot 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. Cook for another 3 minutes. Transfer the custard to the pie crust.
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 completely, and transfer to the oven. Bake until browned, about 10 minutes. Refrigerate until ready to serve.