This bar with a graham cracker crust has old-school charm. Its creamy, cool texture is refreshing, and it is simple to make. To celebrate St. Patrick's Day, substitute key lime juice and zest for the lemon to make a lime-green pie bar.
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.
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.
Makes bout 3 cups