01 - Using an electric mixer on medium-high speed, whip the chilled heavy cream in a mixing bowl until soft peaks form.
02 - Gently fold the sweetened condensed milk into the whipped cream until fully incorporated and smooth.
03 - Arrange graham crackers in a single layer at the bottom of a 9x13-inch glass dish, breaking crackers as needed to fit the space.
04 - Spread one-third of the cream mixture evenly over the graham crackers.
05 - Distribute one-third of the sliced mangoes evenly over the cream layer.
06 - Repeat the layering process twice more: graham crackers, cream mixture, and mangoes, finishing with a mango layer on top.
07 - Sprinkle graham cracker crumbs over the top if desired. Cover tightly with plastic wrap.
08 - Refrigerate for at least 6 hours or overnight until completely set and chilled through.
09 - Slice into squares and serve cold. Garnish with fresh mint leaves if using.