Place a layer of graham crackers, into the bottom of a 9x13 pan. Set the graham crackers side by side to cover the whole bottom of the pan. Break graham cracker pieces as needed to fill the whole pan.
Stir together your dry pudding mix and milk in a large bowl for about 2 minutes till it starts to thicken.
Fold in the lemon zest and 1/2 cup of the cool whip until mixed throughout. Set aside.
Carefully spread half of the pudding mixture over the top of the graham crackers. Spread evenly to the edges.
Place another layer of graham crackers to cover the pudding completely and go to the edges of the pan.
Spread the remaining pudding over the top of the graham crackers, evenly to the edges.
Add final layer of graham crackers, placed evenly over the pudding.
Spread remaining cool whip over the top of the final layer of graham crackers, evenly to the edges.
Cover the cake with plastic wrap and set in the fridge for 4 hours, up to overnight.
**Or place ice box cake in the freezer. Remove from freezer and allow to come to room temperature for about 15 minutes before serving.
Slice and serve.