For the crust
3 cups graham cracker crumbs (approx. one box)
12 Tablespoons unsalted butter, melted
2 Tablespoons sugar
For the filling
1 cup heavy cream
2 teaspoons grated lemon zest
1 cup fresh lemon juice
Two 14-ounce cans sweetened condensed milk
6 large egg yolks
Grease a 9-by-13-by-2-inch baking pan and line with parchment, allowing the ends of the paper to hang over two opposite edges of the pan.
To make the crust: In a medium bowl, combine the graham cracker crumbs, butter and sugar and blend with a fork until the crumbs are evenly moistened. Press the graham cracker mixture firmly and evenly into the bottom of the prepared pan.
Bake the crust for about 8 minutes, until lightly golden. Let cool completely before adding the filling. Turn the oven temperature down to 325°F.
To make the filling: In a large bowl, whisk together the heavy cream, lemon zest, lemon juice, sweetened condensed milk, and egg yolks. Pour the filling over the cooled crust.
Place the baking pan inside a larger baking pan and pour enough hot water into the larger pan to come halfway up the sides of the smaller pan. Bake for 20 to 25 minutes, until the filling is puffed at the edges and no longer jiggles in the center when the pan is tapped. Remove the pan from the water bath and set it on a rack to cool for 1 hour, then refrigerate until cold.
Cut the bars into squares and serve chilled with whipped cream. The bars will keep in an airtight container in the refrigerator for up to 3 days.