01 - Preheat oven to 350°F (175°C).
02 - In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter until evenly moistened. Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish. Bake for 8 to 10 minutes until lightly golden. Set aside to cool completely.
03 - In a medium saucepan, whisk together granulated sugar, cornstarch, all-purpose flour, and salt. Gradually whisk in the water, lemon zest, and lemon juice until smooth. Place over medium heat and stir constantly until the mixture thickens and begins to bubble, about 4 to 6 minutes.
04 - Slowly drizzle a few spoonfuls of the hot lemon mixture into the lightly beaten egg yolks while whisking vigorously to prevent scrambling. Pour the tempered egg mixture back into the saucepan, stirring to combine.
05 - 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 smooth. Pour the filling into the cooled crust.
06 - Allow the pie to cool to room temperature, then refrigerate for at least 2 hours until the filling is fully set.
07 - 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 evenly over the chilled pie.
08 - Slice into 8 portions and serve chilled. Garnish with fresh berries or additional lemon zest if desired.