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.