In a medium bowl, whisk together the sweetened condensed milk, lemon juice, and lemon zest until combined.
Gently fold the thawed whipped topping into the lemon mixture until fully incorporated. This is your lemon cream filling.
Line an 8×8 inch baking pan with plastic wrap, leaving an overhang for easy removal later.
Arrange a single layer of graham crackers on the bottom of the prepared pan. Break crackers if necessary to fit the space.
Spread half of the lemon cream filling evenly over the graham cracker layer.
Place a second layer of graham crackers over the cream.
Spread the remaining lemon cream filling over the second cracker layer.
Top with a final layer of graham crackers.
Cover the pan tightly with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow the crackers to soften and the cake to set.
Before serving, remove the cake from the pan using the plastic wrap overhang.
Garnish the top with extra whipped topping and additional lemon zest, if desired.
Slice and serve cold.
Notes
For the best texture, chill the cake for a full 8 hours or overnight. This ensures the graham crackers become soft like cake layers.
You can use vanilla wafers instead of graham crackers for a different base flavor.
Store leftovers covered in the refrigerator for up to 3 days.