Preheat oven to 350°F. Pour the graham cracker crumbs into a bowl and stir in the melted butter and sugar. Once all the graham cracker crumbs are moistened, press the crumb mixture into the bottom and halfway up the sides of a 9-inch pie plate. Bake the crust for about 10-12 minutes, until fragrant and light golden brown. Set aside to cool.

In a heavy saucepan, combine sugar and cornstarch. Whisk in milk and lemon juice. Place the pan over medium-high heat and cook, stirring frequently, until thickened and bubbling. Reduce heat to low and cook for 2 more minutes, stirring frequently. Remove from the heat.

Place the egg yolks in a bowl. Slowly add one generous cup (eyeball it) of the hot mixture into the egg yolks, whisking as you pour. Add the egg mixture back to the pan and bring the whole thing to a gentle boil for 1-2 minutes, whisking constantly. Remove from the heat. Stir in butter and lemon zest, mixing until completely combined. Transfer to a bowl and cool to room temperature. This takes about one hour.

Once cooled, whisk in sour cream. Whisk vigorously to maintain the creamy texture. Pour the lemon filling into the cooled crust. Cover and refrigerate until chilled, at least 6 hours.

Before serving, whip heavy cream, powdered sugar, and vanilla extract until stiff peaks form. Top the pie with whipped cream. You can spread it or pipe it using a star tip and piping bag. If desired, garnish with a sprinkling of finely grated lemon zest.