Cut a piece of parchment paper into a sling and place it into a 9x13 baking dish. You want it to come up over the two long sides as this will help you to remove the bars later.
Add your graham crackers to a food processor. Process until fine crumbs.
Add in the lemon zest, melted butter, and sugar. Process again until combined.
Pour the graham cracker mixture into the prepared baking dish.
Press out into a crust. Bake in the oven at 325F for 5 minutes.
In a medium-sized pot on the stove, over medium heat, add the flour, lemon juice, and sugar.
Cook and stir until the sugar has dissolved and the mixture looks smooth.
Crack your eggs and place them into a large mixing bowl. Whisk until the eggs are broken.
Temper the eggs by adding the hot lemon juice mixture to the eggs, a little at a time. Whisk well after each addition.
Continue until all of the lemon mixture has joined the eggs in the bowl.
Pour the contents of the bowl into the pot.
Continue to cook while frequently whisking until the mixture begins to thicken. Remove from heat.
Place your baked graham cracker crust in front of you. If you turned the oven off, preheat it back to 325 degrees F.
Pour the lemon filling on top. Carefully spread out into a nice even layer. You don't want to disturb the crust.
Bake in the oven at 325F for 25 minutes.
Remove from oven and let cool for 1-2 hours at room temperature before placing in the fridge for another 1-2 hours (or longer).
Slice into serving-sized squares. Dust with powdered sugar and enjoy.
Notes
Variations:
For a gluten-free version, use gluten-free flour and graham crackers.
Recipe Tips:
Crush graham crackers in a sealed plastic bag with a rolling pin if you don't have a food processor.
Wipe the knife with a clean towel after each slice for clean cuts.
You can substitute lemon juice with lemon juice concentrate.
Storage:
Refrigerate covered lemon bars for up to 1 week, best within 3-4 days.
Freeze in an airtight container for up to 3 months. Thaw in the fridge and dust with powdered sugar before serving.