01 - Preheat oven to 350°F. In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter until evenly moistened. Press the mixture firmly into the bottom and along the sides of a 9-inch pie dish to form an even crust.
02 - Bake the crust for 8 to 10 minutes until lightly golden and set. Remove from the oven and allow it to cool completely while preparing the filling.
03 - In a medium saucepan, whisk together granulated sugar, cornstarch, all-purpose flour, and salt until no lumps remain. Gradually whisk in the water, lemon zest, and freshly squeezed lemon juice until smooth.
04 - Place the saucepan over medium heat and stir constantly for 4 to 6 minutes, until the mixture thickens and begins to bubble gently.
05 - While whisking the egg yolks continuously, slowly drizzle in a few spoonfuls of the hot lemon mixture to gradually raise the temperature of the yolks without scrambling them. Pour the tempered egg mixture back into the saucepan, stirring to combine.
06 - Continue cooking the filling over medium heat for 2 minutes, stirring constantly, until thick and glossy. Remove from heat and stir in the butter until fully melted and the filling is smooth.
07 - Pour the warm lemon filling into the cooled crust, spreading it evenly. Allow the pie to cool to room temperature, then refrigerate for at least 2 hours until fully set.
08 - Using an electric mixer, beat the heavy whipping cream, powdered sugar, and vanilla extract on medium-high speed until stiff peaks form. Spread or pipe the whipped cream over the chilled pie.
09 - Slice the pie into 8 even portions and serve chilled. Garnish with fresh berries or extra lemon zest if desired.