In a large bowl, whip the heavy cream until soft peaks form.
In another bowl, mix the sweetened condensed milk, lemon juice, lemon zest, and vanilla extract until smooth.
Gently fold the whipped cream into the lemon mixture until fully combined and fluffy.
In a serving dish, spread a thin layer of the cream mixture.
Add a layer of graham crackers or cookies over the cream.
Repeat layering with cream and cookies until all ingredients are used, finishing with a cream layer on top.
Cover and refrigerate for at least 6 hours, preferably overnight.
Garnish with extra lemon zest before serving, slice, and enjoy chilled.