01 - Preheat oven to 350°F.
02 - Combine graham cracker crumbs, granulated sugar, and melted butter in a mixing bowl 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 beaten egg yolks while whisking vigorously to temper. Pour the warmed egg mixture back into the saucepan, stirring continuously. Cook for an additional 2 minutes until the filling is thick and glossy.
05 - Remove the saucepan from heat and stir in the butter until fully melted and smooth. Pour the lemon filling into the cooled crust. Allow the pie to cool to room temperature, then refrigerate for at least 2 hours until firmly set.
06 - 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.
07 - Cut into slices and serve chilled. Ensure the pie is fully set before slicing for clean portions.