Once it is starting to thicken, add 1 tablespoon of lemon zest & ¼ cup fresh lemon juice. Whisk to combine.
To the lemon pudding mix, fold in half of the defrosted Cool Whip (8 ounces or 1 small tub). Do this gently so it mixes up and stays light - it is ok if there are a few small lumps.
Place the pudding in the refrigerator to keep it cold & help it thicken while doing the next steps.
To the remaining 8 ounces of Cool Whip, add a tabelspoon of lemon zest & fold to combine, set aside.
To the bottom of the 9x13 pan, add a layer of graham crackers. Break any so they fit & the bottom is covered.
Top the graham crackers with half of the pudding mix. Spread to cover all edges and corners.
Repeat with another layer of graham crackers over the pudding. Then top the 2nd layer of crackers with the final half of the pudding. Add a 3rd and final layer of graham crackers.
Top with the Cool Whip mixture. Spread and swirl so everything is covered. Sprinkle on any remaining lemon zest.
Place the icebox cake in the refrigerator. Chill for 4 hours minimum, overnight is best.
Cut the cake into squares, serve, & enjoy immediately. Keep leftovers in the refrigerator at all times so it doesn't soften.
Notes
STORING:
Store leftovers in the fridge for up to 5 days.
Place in the freezer for 1-2 hours for it to become a bit harder to create clean even slices. I love it this way & the extra cold texture is amazing.
SUBSTITUTIONS:
Use gluten-free graham crackers. Try nilla wafers or other thin biscuit cookies instead of regular graham crackers.
Use store-bought lemon juice instead of freshly squeezed.