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