- In a medium saucepan, combine 1 cup granulated sugar, 1/4 cup cornstarch, and 1/8 teaspoon salt. Slowly whisk in 2 cups whole milk until smooth. Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and comes to a gentle boil, about 5-7 minutes. 
- In a separate bowl, lightly beat the 4 large egg yolks. Gradually pour about 1 cup of the hot milk mixture into the yolks while whisking constantly to temper them. Then pour the yolk mixture back into the saucepan with the remaining milk mixture. 
- Return the saucepan to medium heat and cook for another 2 minutes, stirring constantly until the filling is thick and glossy. Remove from heat and stir in 1/2 cup fresh lemon juice, zest of 2 lemons, 2 tablespoons unsalted butter, and 1 teaspoon vanilla extract until combined. 
- Pour the lemon filling into the pre-made graham cracker pie crust. Smooth the top with a spatula. Cover the pie with plastic wrap, pressing lightly against the surface to prevent a skin from forming. Refrigerate for at least 4 hours or until fully set. 
- In a chilled mixing bowl, beat 1 cup heavy whipping cream with 2 tablespoons powdered sugar until stiff peaks form. Spread the whipped cream evenly over the chilled lemon filling. 
- Optionally, garnish with lemon zest or thin lemon slices. Slice and serve chilled. Enjoy the refreshing and creamy Melt-In-Your-Mouth Lemon Cream Pie.