Position a rack in the middle of the oven and preheat the oven to 350 degrees F. 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. Use a piece of parchment to press on the crust with the palm of your hands to make sure it is completely level.
Bake the crust for about 8 minutes, until lightly golden. Let cool completely before adding the filling. Turn the oven temperature down to 325 degrees 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 the whipped cream. The bars will keep in an airtight container in the refrigerator for up to 3 days.
Fresh Whipped Cream: Using a stand mixer fitted with the whisk attachment (or in a large mixing bowl, using a handheld mixer), whip the cream on medium speed until it starts to thicken. Add the confectioners' sugar and beat until the cream holds nice soft peaks. Makes bout 3 cups.
Photographs by Squire Fox.
From THE BACK IN THE DAY BAKERY COOKBOOK by Cheryl Day and Griffith Day, coming from Artisan Books in March 2012.