In a large mixing bowl, combine sweetened condensed milk, lemon juice, and lemon zest. Stir until well mixed. In a separate chilled bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the lemon mixture.

Spread a thin layer of the lemon filling at the bottom of a 9-inch round cake pan. Place a layer of graham crackers over the filling. Add another layer of lemon filling over the crackers and repeat until all ingredients are used, finishing with a layer of filling on top.

Cover the cake pan with plastic wrap and refrigerate for at least 4 hours or overnight. Before serving, garnish with additional lemon zest or berries as desired. Slice into 8 servings and enjoy.

Cover leftover cake with plastic wrap and store in the refrigerator for up to 3 days. For longer storage, wrap tightly and freeze for up to a month. Thaw in the refrigerator before serving.