Prepare the pudding layer: In a large bowl, whisk the lemon pudding mix with cold milk until smooth and thickened. Set aside.
Make the cream cheese layer: In a separate bowl, beat the softened cream cheese until creamy. Add powdered sugar and vanilla extract, and beat again until smooth.
Fold in whipped topping: Gently fold the whipped topping into the cream cheese mixture until fully combined. Stir in lemon zest.
Layer the cake: In a 9×13 inch dish, place a single layer of graham crackers at the bottom. Spread half of the cream cheese mixture evenly over the crackers.
Add pudding layer: Spread half of the lemon pudding over the cream cheese layer.
Repeat layers: Add another layer of graham crackers, then the remaining cream cheese mixture, followed by the remaining lemon pudding.
Chill the cake: Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight, allowing the graham crackers to soften into a cake-like texture.
Garnish and serve: Before serving, top with extra whipped topping, lemon zest, and sliced lemons if desired. Slice and enjoy chilled!