01 - Preheat oven to 350°F. In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter until the mixture resembles wet sand and holds together when pressed.
02 - Press the crumb mixture firmly and evenly 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 until no lumps remain. Gradually whisk in the water, lemon zest, and 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 - 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.
06 - Continue cooking and stirring for 2 minutes until the filling is thick, glossy, and smooth. Remove from heat and stir in the butter until fully melted and incorporated.
07 - Pour the hot lemon filling into the completely cooled crust, spreading 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 fully chilled pie into 8 even portions and serve immediately. Garnish with fresh berries or extra lemon zest if desired.