Grease and line a 9x13 pan with parchment paper, leaving a slight overhang on each side so that you can lift them out later. Set aside.
In a large bowl, mix together graham cracker crumbs (3 cups) and melted butter (1 cup, i.e. 2 sticks). Press firmly into the prepared pan to create the crust.
In a separate large bowl, add softened cream cheese (32-ounces, i.e. four 8-ounce packs), sweetened condensed milk (two 14-ounce cans), lemon juice (⅓ cup), and lemon zest (3 tablespoons). Use an handheld or stand mixer to beat mixture for 2-3 minutes until smooth and creamy.
Pour filling on top of the graham cracker crust and smooth with a spatula. Gently tap the pan on the counter to remove any air bubbles.
Cover and refrigerate (or freeze) for 4-6 hours, or until firm (overnight is even better!). If it is still not firm after that time in the refrigerator, pop it in the freezer. Starting with the bars really cold will ultimately give you the cleanest cuts, so I recommend popping it into the freezer for the last hour of chilling to really firm things up!
When ready to serve, remove from the refrigerator or freezer and top with extra lemon zest, if desired. Cut into 12-24 squares. Tip: To get clean slices, use your parchment overhang to lift the chilled bars out of the pan. Dip a large straight-edged knife into a tall cup of hot water then wipe it off between each cut. When cutting, press the knife down and then slide it out the side, rather than lifting it back up. For added decoration, top each square with a dollop of whipped cream, fresh strawberries, or a lemon slice right before serving. Serve chilled.