In a large mixing bowl, whisk the sweetened condensed milk, lime juice and zest.
In a separate large bowl, beat 2 ¼ cups of cold heavy cream with 1tsp vanilla until soft peaks form.
Fold ⅓ whipped cream mixture into the sweetened condensed milk mixture.
Repeat 2 more times, folding in ⅓ of the whipped cream mixture each time.
Add a layer of graham crackers to a 9x13 baking dish, covering the entire bottom.
Break apart to fill spaces if needed.
Spread ½ the cream mixture over the graham crackers.
Add another layer of graham crackers.
Spread remaining cream mixture on top.
Top with remaining graham crackers.
Set in the fridge while you make the rest of the whipped cream.
In another large bowl, beat the remaining 1 ½ cups of cold heavy cream with 6 tbsp sugar and 1 tsp vanilla until soft peaks form.
Add the final layer of whipped cream over the ice box cake.
Cover with plastic wrap and return to the refrigerator overnight.
Top with lime zest and enjoy!
