In a medium bowl, whisk together the sweetened condensed milk, fresh lemon juice, and lemon zest until completely smooth and slightly thickened. The mixture will thicken significantly due to the acid in the lemon juice reacting with the milk.

Gently fold the thawed whipped topping into the lemon mixture using a rubber spatula until no white streaks remain. Be careful not to overmix, as you want to maintain the fluffiness.

Line an 8x8 inch baking dish with plastic wrap, allowing excess to drape over the sides for easy removal later. Arrange a single layer of graham crackers on the bottom of the dish, breaking them as needed to fit snugly.

Spread half of the lemon cream mixture evenly over the first layer of graham crackers. Top with a second layer of graham crackers, followed by the remaining lemon cream mixture. Finish with a final layer of graham crackers.

Cover the dish tightly with plastic wrap and refrigerate for a minimum of 6 hours, or preferably overnight, allowing the crackers to soften into a cake-like consistency.

Before serving, carefully lift the cake out of the dish using the plastic wrap overhang. Dust the top lightly with powdered sugar, slice into squares, and serve cold.