01 - Whip the chilled heavy cream with an electric mixer on medium-high speed until soft peaks form.
02 - Gently fold in the sweetened condensed milk until well combined and smooth.
03 - Arrange a single layer of graham crackers in a 9x13-inch glass dish, breaking as needed to fit.
04 - Spread a third of the cream mixture evenly over the graham crackers.
05 - Layer with a third of the sliced mangoes, distributing them evenly.
06 - Repeat the layers twice more: crackers, cream, mangoes, finishing with mangoes on top.
07 - Sprinkle reserved graham cracker crumbs on top for garnish, if desired.
08 - Cover with plastic wrap and refrigerate for at least 6 hours, or overnight, until set.
09 - Slice and serve cold. Garnish with mint leaves if desired.