01 - Preheat your oven to 350°F (175°C). Position the rack in the center of the oven.
02 - In a mixing bowl, combine the graham cracker crumbs, granulated sugar, and melted butter. Stir until the mixture resembles wet sand and holds together when pressed. Transfer to a 9-inch pie dish and press firmly into the bottom and up the sides to form an even crust.
03 - Bake the crust for 8–10 minutes until lightly golden and fragrant. Remove from the oven and let it cool completely on a wire rack.
04 - In a medium saucepan, whisk together the granulated sugar, cornstarch, all-purpose flour, and salt until evenly combined. Gradually pour in the water, lemon zest, and lemon juice, whisking constantly to prevent lumps.
05 - Place the saucepan over medium heat and stir continuously for 4–6 minutes until the mixture thickens and begins to gently bubble.
06 - Slowly drizzle a few spoonfuls of the hot lemon mixture into the beaten egg yolks while whisking vigorously to temper them. Pour the warmed egg mixture back into the saucepan, stirring to incorporate.
07 - Continue cooking the filling over medium heat for 2 minutes, stirring constantly, until it becomes thick and glossy. Remove from heat and stir in the butter until fully melted and smooth.
08 - Pour the hot 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.
09 - 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.
10 - Slice the pie into 8 even portions and serve chilled. Garnish with fresh berries or additional lemon zest if desired.