¾cupplus 2 tablespoons lemon juice, from about 5-6 lemons
1cupheavy whipping cream
Preheat the oven to 325 degrees F making sure one of the racks is in the middle of the oven. Break the graham crackers into pieces and add them to a blender, food processor or ziploc bag and crush (or roll the bag with a rolling pin) until the crackers are finely ground. Mix in the sugar and butter (if using a ziploc bag, transfer the cracker crumbs to a bowl before adding ingredients).
Press the crumb mixture into the bottom and up the sides of a 9-inch pie plate. Bake the crust for 12-14 minutes, until it is just beginning to brown. Cool the crust completely on wire rack.
Once the crust is cool, preheat the oven to 375 degrees F.
Measure out 3 tablespoons of the sweetened condensed milk and set aside in a small container (it will be used later). Whisk the remaining condensed milk with the egg yolks until smooth. Slowly whisk in the lemon juice. Pour the filling into the cooled crust and bake the pie until the edges are just set but the center still jiggles when the pie plate is lightly moved, about 15 minutes.
Let the pie cool completely on a wire rack and then cover and refrigerate until well-chilled and set, about 3 hours or up to 24 hours.
When ready to serve, using an electric stand mixer or handheld mixer, whip the cream, reserved condensed milk and vanilla until stiff peaks form. Spread the whipped topping over the pie. At this point, the pie could be chilled for up to an hour (any longer and the whipped cream topping can cause the pie to 'weep' and get liquidy). Cut into slices and serve.
Make Ahead: the crust and pie combo can be made and refrigerated for up to 24 hours in advance (but don't top the pie with the whipped cream until ready to serve or less than an hour before serving).